Inkjet recording method and inkjet printer

ABSTRACT

An inkjet recording method having: jetting recording ink containing a color material onto a recording medium by a recording head, and colorless ink for improving gloss onto the recording medium by the recording head, to perform image formation; and determining an adhered amount of the colorless ink per unit area in response to an adhered amount of the recording ink per unit area.

FIELD OF THE INVENTION

The present invention relates to an inkjet recording method and aninkjet printer, and particularly, to an inkjet recording method and aninkjet printer, which are for jetting ink for improving gloss onto arecording medium.

BACKGROUND ART

An inkjet printer for performing image formation by jetting minutedroplets of ink onto a recording surface of a recording medium has beenrapidly widespread because higher image quality approaching that of asilver-halide photograph and price reduction of the apparatus have beenrealized therein thanks to a technological progress in recent years.

In the conventional inkjet printer, dye ink has been mainly used. Thedye ink is soluble in a solvent, exhibits high-purity and clear colorreproduction, and is free from granularity. Accordingly, the dye inkdoes not cause scattered light or reflected light, has high transparencyand a clear hue. Therefore, the dye ink is excellent at printing ahigh-quality image. On the contrary, when molecules of the coloringmatter are destroyed owing to a photochemical reaction and the like, adecrease of the number of molecules directly affects a colored density,and accordingly, there is a problem that the dye ink is poor in lightresistance.

While on the other hand, other colorants advantageous in lightresistance, ozone resistance and the like have been proposed. Arepresentative example is pigment ink that has already been put intopractical use, and further, the colorants include coloring particlesformed by containing lipophilic dye such as dispersed dye in resin, waxink (hot melt ink) formed by dispersing or dissolving a color materialinto wax, and the like.

Moreover, a recording medium in which gloss is extremely high, such as aswelling-type one mainly containing a water-soluble binder, is used as arecording medium. Alternatively, a mixed layer of thermoplastic fineparticles and inorganic fine particles is provided on a surface layer ofa micro-porous recording medium, and after an image is formed by usingthe pigment ink, a post treatment such as fixing with heat and pressureis performed for the image, and so on (for example, refer to EuropeanPatent Laid-Open Specification No. 1228891 as Patent Document). In sucha way, high image gloss is adapted to be obtained.

However, in many cases, the pigment ink and the wax ink contain resintherein in order to improve dispersibility of the coloring particles, toimprove rubfastness of the image, and to improve the image gloss. Whenthe image formation is performed by using such ink, gloss of ahighlighted portion in which image-unformed regions are more thanimage-formed regions and of a blank portion is lowered, and a feeling ofwrongness occurs in the gloss as a whole. Specifically, it is stronglydesired to eliminate the feeling of wrongness by solving unevenness ofthe gloss as a whole of the recording medium.

Moreover, in the case of using the swelling-type recording medium, inkabsorption speed thereof is insufficient for a high-speed image formingprinter in recent years, and accordingly, it is difficult to achievecompatibility between ink absorptivity and a solution of the low glossinherent in the highlighted portion in which the image-unformed regionsare many and in the blank portion.

Further, in the case of using the recording medium described in theabove-described Patent Document, a certain amount of inorganic fineparticles will be used in order to obtain sufficient ink absorptivity,and accordingly, the gloss of the highlighted portion and the blankportion is lowered than the image-formed regions.

It is an object of the present invention to suppress the unevenness ofthe gloss of the image-formed regions, the highlighted portion and theblank portion, thereby solving the feeling of wrongness.

DISCLOSURE OF THE INVENTION

To solve the above problems, the inkjet recording method comprises:

jetting recording ink containing a color material onto a recordingmedium by a recording head, and colorless ink for improving gloss ontothe recording medium by the recording head, to perform image formation;and

determining an adhered amount of the colorless ink per unit area inresponse to an adhered amount of the recording ink per unit area.

Moreover, the inkjet printer, comprises:

an image forming unit to jet recording ink containing a color materialonto a recording medium by a recording head, and jet colorless ink forimproving gloss onto the recording medium by the recording head, therebyperforming image formation; and

a control unit to control the image forming unit, wherein the controlunit determines an adhered amount of the colorless ink per unit area inresponse to an adhered amount of the recording ink per unit area.

According to the inkjet recording method and the inkjet printer of thepresent invention, the recording ink is jetted onto the recordingmedium, the colorless ink for improving the gloss is jetted onto therecording medium, and the image formation is thus performed.Accordingly, even if the gloss of the image-formed regions is improvedby the color material contained in the recording ink, the colorless inkis jetted onto the blank portion and the highlighted portion in whichthe adhered amount of recording ink is small, thus making it possible toimprove the gloss of these portions. In particular, the adhered amountof colorless ink per unit area is determined in response to the adheredamount of recording ink per unit area. Accordingly, the gloss on therecording surface of the recording medium can be made even, and thefeeling of wrongness owing to the unevenness of the gloss can be solved.

Moreover, the colorless ink for use in the present invention stands forink that does not substantially contain the color material, and it ispreferable that a variation of ΔE of the image portion, which is causedby the existence of the colorless ink, be 3 or less. A gloss-impartingfunction to be referred to herein means a function to improve speculargloss (JIS-Z-8741) and image clarity (JIS-K-7105).

(Measurement of 60-Degree Gloss)

A 60-degree specular gloss of an image-formed surface was measured inaccordance with JIS-Z-8741. For the measurement, a variable angle glosssystem (VGS-1001DP) made by Nippon Denshoku Industries Co., Ltd. wasused.

Moreover, in the inkjet recording method of the present invention, ajetted position of the colorless ink may be determined in response to ajetted position of the recording ink.

Meanwhile, in the inkjet printer of the present invention, the controlunit may determine a jetted position of the colorless ink in response toa jetted position of the recording ink.

As described above, the jetted positions of the colorless ink aredetermined in response to the jetted positions of the recording ink, andaccordingly, it is made possible to jet the colorless ink onto suitablepositions with respect to the jetted positions of the recording ink.

Moreover, in the inkjet recording method of the present invention, thejetted position of the colorless ink may be determined preferentiallyfrom a position that is not adjacent to or overlapped on the jettedposition of the recording ink.

Meanwhile, in the inkjet printer of the present invention, the controlunit may determine the jetted position of the colorless inkpreferentially from a position that is not adjacent to or overlapped onthe jetted position of the recording ink.

As described above, the jetted positions of the colorless ink aredetermined preferentially from the positions which are not adjacent toor overlapped on the jetted positions of the recording ink, andaccordingly, the colorless ink and the colorless ink can be preventedfrom being mixed with each other on the recording medium.

Moreover, in the inkjet recording method of the present invention, theadhered amount of colorless ink may be increased in a region where theadhered amount of recording ink is a predetermined amount or less thanin a region where the adhered amount of recording ink is more than thepredetermined amount.

Meanwhile, in the inkjet printer of the present invention, the controlunit may increase the adhered amount of the colorless ink in a regionwhere the adhered amount of the recording ink is a predetermined amountor less than in a region where the adhered amount of the recording inkis more than the predetermined amount.

As described above, on the region where the adhered amount of recordingink is equal to or less than the predetermined amount, the adheredamount of colorless ink is increased in comparison with that of theregion where the adhered amount of recording ink is more than thepredetermined amount. Accordingly, on the region where the adheredamount of recording ink is more than the predetermined amount, theadhered amount of colorless ink is reduced than that of the region wherethe adhered amount of recording ink is equal to or less than thepredetermined amount. Then, ink of which amount exceeds a permissibleink absorption amount of the recording medium becomes less prone to bejetted. Hence, a liquid overflow which is caused because the recordingmedium cannot fully absorb the ink can be prevented.

Moreover, in the inkjet recording method of the present invention, theunit area for the adhered amounts of the colorless ink and the recordingink may be set at 1 mm square or less, and a sum total of the adheredamounts of the colorless ink and the recording ink in the unit area maybe set at a predetermined amount or more.

Meanwhile, in the inkjet printer of the present invention, the controlunit may set the unit area for the adhered amounts of the colorless inkand the recording ink at 1 mm square or less, and set a sum total of theadhered amounts of the colorless ink and the recording ink in the unitarea at a predetermined amount or more.

In general, the adhered amount of ink in the inkjet recording methodstands for an adhered amount thereof per fixed area of the recordingmedium, that is, an adhered amount thereof per unit area. The unit inthis case is the overall surface area of the recording medium at themaximum, and is one pixel corresponding to a recording resolution at theminimum. In the case of improving evenness of characteristics of therecording medium, even if the adhered amount of colorless ink iscontrolled with the full recording surface taken as a unit, an effectbrought therefrom is small, and this is obvious. Hence, a certainmaximum value exists as a unit to be controlled in the case of jettingthe colorless ink, and it is desirable to employ a value equal to orless than the maximum value concerned as the unit to be controlled.

As a result of a study of the inventor of the present invention, it hasbeen found that, in the case of improving the evenness of the gloss ofthe recording surface, 2 mm or less is essential as the maximum unit tobe controlled, and 0.5 mm square is more preferable. Resolving power ofa human eye has the highest sensitivity at an interval of 0.5 mm when adistance of the eye to the recording medium is set at approximately 30cm. Hence, in the case of ensuring evenness of a black density of therecording surface by means of dots of a recording head, it is necessarythat the dots be distributed at a spatial frequency higher than theabove.

Moreover, in the case of a printed matter with high image quality, a dotinterval (so-called screen ruling) becomes 150 to 175, and a spatialfrequency thereof has an interval of 0.169 to 0.145 mm.

However, it has been found that the resolving power of the human eye isnot very high for characteristics such as the gloss, and that a verylarge feeling of wrongness does not occur even if portions with thegloss and portions without the gloss are distributed uniformly at aninterval of approximately 1 mm. Specifically, according to theabove-described inkjet recording method and inkjet printer, the unitarea of the adhered amounts of colorless ink and recording ink is set at1 mm square, and the sum total of the adhered amounts of colorless inkand recording ink within the unit area is set at the predeterminedamount or more. Accordingly, the uniformity of the gloss on therecording surface can be enhanced more, and the feeling of wrongnessowing to the unevenness of the gloss can be solved. Note that, thoughhigher evenness can be obtained if a range finer than 1 mm square isemployed as the unit area, it is obvious that it is the most efficientto make the control by means of a necessary and sufficient size unit inconsideration of a calculation time required for the dot distribution.

Moreover, in the inkjet recording method of the present invention, thesum total of the adhered amounts of the colorless ink and the recordingink in the unit area may be 2 cc/m² or more.

Meanwhile, in the inkjet printer of the present invention, the controlunit may set the sum total of the adhered amounts of the colorless inkand the recording ink in the unit area at 2 cc/m² or more.

As described above, the sum total of the adhered amounts of colorlessink and recording ink within the unit area is set at 2 cc/m² or more.Accordingly, the uniformity of the gloss on the recording surface can beenhanced more stably.

Moreover, in the inkjet recording method of the present invention, thesum total of the adhered amounts of the colorless ink and the recordingink in the unit area may be less than 13 cc/m².

Meanwhile, in the inkjet recording apparatus of the present invention,the control unit may set the sum total of the adhered amounts of thecolorless ink and the recording ink in the unit area at less than 13cc/m².

Here, when the sum total of the adhered amounts of the colorless ink andthe recording ink within the unit area is set at 13 cc/m² or more, thereis a possibility that the sum total exceeds the permissible inkabsorption amount of the recording medium to cause the liquid overflow.Therefore, as in the above-described inkjet recording method and inkjetprinter, if the sum total of the adhered amounts of the colorless inkand the recording ink within the unit area is set at less than 13 cc/m²,the liquid overflow can be prevented.

Moreover, in the inkjet recording method of the present invention, theunit area for the adhered amounts of the colorless ink and the recordingink may be set as a block formed of an aggregate of n (n>1) pieces ofpixels.

Meanwhile, in the inkjet printer of the present invention, the controlunit may set the unit area for the adhered amounts of the colorless inkand the recording ink as a block formed of an aggregate of n (n>1)pieces of pixels.

In general, in the case of printing an image having gradation, such as aphotograph, by an inkjet mode, the number of gradations for each pixelis short, and accordingly, a halftone process using error diffusion anda dither matrix becomes necessary. In this case, when the area of theunit to be controlled is set as a unit of the dither matrix, data forthe colorless ink can be calculated simultaneously with the halftoneprocess, and this is efficient. In particular, the dither matrix is atechnique for use when a high-speed output is desired though the imagequality is not required very much, where an effect that the calculationof the adhered amount of colorless ink can be made at the high speed islarge. The dither matrix is one taking a×b (=n) pixels in a usual imageas one block and using the block as a unit for determining dotformation, where a is the number of pixels in the lateral direction andb is the number of pixels in the longitudinal direction. Specifically,as in the above-described inkjet recording method and inkjet printer, ifthe unit area of the adhered amounts of the colorless ink and therecording ink is set as the block formed of an aggregate of the n (n>1)pieces of pixels, the adhered amounts of the recording ink and thecolorless ink can be controlled correspondingly to the dither matrix.

Moreover, in the inkjet recording method of the present invention, ajetted position of the colorless ink jetted onto the block may bedetermined preferentially from a pixel in which the adhered amount ofthe recording ink is smaller.

Meanwhile, in the inkjet printer of the present invention, the controlunit may determine a jetted position of the colorless ink jetted ontothe block preferentially from a pixel in which the adhered amount of therecording ink is smaller.

As described above, the jetted position of the colorless ink jetted intothe block is determined from the pixel in which the adhered amount ofrecording ink is smaller, and accordingly, the colorless ink can bejetted preferentially from a pixel on which the recording ink is notjetted, and this is effective from viewpoints of the ink overflow andthe evenness of the gloss.

Moreover, in the inkjet recording method of the present invention, theunit area for the adhered amounts of the colorless ink and the recordingink may be defined as one pixel, and a sum total of the adhered amountsof the colorless ink and the recording ink in the unit area may be setat a predetermined amount or more.

Meanwhile, in the inkjet printer of the present invention, the controlunit may define the unit area for the adhered amounts of the colorlessink and the recording ink as one pixel, and set a sum total of theadhered amounts of the colorless ink and the recording ink in the unitarea at a predetermined amount or more.

As described above, the unit area of the adhered amounts of thecolorless ink and the recording ink is set at one pixel, and the sumtotal of the adhered amounts of colorless ink and recording ink withinthe unit area is set at the predetermined amount or more. Accordingly,it is made possible to determine the adhered amount of colorless ink foreach pixel.

Meanwhile, in the determination of the jetted position of the colorlessink, in the case of obtaining the jetted position concerned based on ajetted position of the recording ink after the halftone process, it iseasy to determine the jetted position of the colorless ink based on thepredetermined adhered amount by the block unit as described above.However, according to the above-described inkjet recording method andinkjet printer, it is made possible to calculate the adhered amount ofcolorless ink by using image data before the halftone process, andaccordingly, the jetted position of the colorless ink can be calculatedby performing the same process as the halftone process.

Moreover, in the inkjet recording method of the present invention, therecording ink may contain fine particles.

Meanwhile, in the inkjet printer of the present invention, the recordingink may contain fine particles.

The effect of improving the evenness of the gloss according to thepresent invention is effective in a combination of the recording mediumand the ink, in which the recording ink is adhered onto the recordingmedium and the gloss is thus improved. The respective materialsthemselves are not limited at all. However, a particular effect isbrought to a system using materials significantly exhibiting the abovephenomenon. The materials as described above include those in the caseof containing fine particles other than the color material in therecording ink itself, the case where the color material itself is fineparticles, and a combination of these two cases, as in theabove-describe inkjet recording method and inkjet printer.

Moreover, in the inkjet recording method of the present invention, therecording medium may include a micro-porous recording medium.

Meanwhile, in the inkjet printer of the present invention, the recordingink may contain fine particles.

As described above, the recording medium is the micro-porous recordingmedium, and accordingly, has higher ink absorption speed than theswelling-type recording medium, and can absorb the ink correspondinglyto the image formation speed of the high-speed image forming printer. Insuch a way, it is made possible to make the gloss and the inkabsorptivity compatible with each other.

Moreover, in the inkjet recording method of the present invention, asurface layer of the recording medium may contain a thermoplastic resin.

Meanwhile, in the inkjet printer of the present invention, the recordingmedium may include a micro-porous recording medium.

As described above, the surface of the recording medium contains thethermoplastic resin. Accordingly, an effect thereof is particularly highin the case of performing heating or fixing with pressure after therecording, further, in the case of using the above-described ink andmedium in combination, and so on.

Moreover, in the inkjet recording method of the present invention, afixing process including heating or pressurization may be implementedfor the recording medium on which the recording ink and the colorlessink are jetted.

Meanwhile, in the inkjet printer of the present invention, a fixingprocess including heating or pressurization may be implemented for therecording medium on which the recording ink and the colorless ink arejetted.

When the surface layer of the recording medium contains thethermoplastic resin, if the recording ink and the colorless ink whichare adhered onto the recording medium are melted or formed into acoating film, more excellent gloss can be obtained. Specifically,according to the above-described inkjet recording method and inkjetprinter, the recording ink and the colorless ink are fixed onto therecording medium by a fixing process including the heating and thepressurization, and these inks can be thus melted or formed into thecoating film, and the gloss can be further improved.

Moreover, in the inkjet recording method of the present invention, arate of light absorbance change in mixing the recording ink and thecolorless ink with each other may be less than 5%.

Meanwhile, in the inkjet printer of the present invention, a rate oflight absorbance change in mixing the recording ink and the colorlessink with each other may be less than 5%.

If the rate of the light absorbance change in mixing the recording inkand the colorless ink with each other is 5% or more, for example, whenthe recording ink and the colorless ink are mixed with each other in anozzle of the recording head, there is a possibility that the nozzle isclogged, bringing a lowering of the image quality and a lowering of thegloss as a result. Therefore, if the rate of the light absorbance changein mixing the recording ink and the colorless ink with each other is setat less than 5% in advance as in the above-described inkjet recordingmethod and inkjet printer, the nozzle can be prevented from beingclogged, and the lowering of the image quality and the lowering of thegloss can be prevented.

Moreover, the inkjet recording method of the present inventioncomprises:

jetting recording ink containing a color material onto a recordingmedium by a recording head, and a colorless ink for improving gloss ontothe recording medium by the recording head, to perform image formation,

wherein a rate of light absorbance change in mixing the recording inkand the colorless ink with each other is less than 5%.

Meanwhile, the inkjet printer of the present invention comprises:

an image forming unit to jet recording ink containing a color materialonto a recording medium by a recording head, and jet colorless ink forimproving gloss onto the recording medium by the recording head, therebyperforming image formation; and

a control unit to control the image forming unit,

wherein a rate of light absorbance change in mixing the recording inkand the colorless ink with each other is less than 5%.

As described above, the recording ink is jetted onto the recordingmedium, the colorless ink for improving the gloss is jetted onto therecording medium, and the image formation is thus performed.Accordingly, even if the gloss of the image-formed regions is improvedby the color material contained in the recording ink, the colorless inkis jetted onto the blank portion and the highlighted portion in whichthe adhered amounts of recording ink are small, thus making it possibleto improve the gloss of these portions. Moreover, the rate of the lightabsorbance change in mixing the recording ink and the colorless ink witheach other is set at less than 5%, and accordingly, the nozzle can beprevented from being clogged, and the lowering of the image quality andthe lowering of the gloss can be prevented.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing main constituents of an inkjetprinter according to a first embodiment.

FIG. 2 is an enlarged perspective view of a carriage provided in theinkjet printer of FIG. 1.

FIG. 3 is a lower surface view of a recording head mounted on thecarriage of FIG. 2.

FIG. 4 is a front view showing main constituents of a fixing unitprovided in the inkjet printer of FIG. 1.

FIG. 5 is a block diagram showing a control circuit of the inkjetprinter of FIG. 1.

FIG. 6 is a waveform diagram showing waveforms of voltages for drivingthe recording head of FIG. 3.

FIG. 7 is a block diagram showing main control unit of an image formingapparatus connected to the inkjet printer of FIG. 1.

FIG. 8 is a flowchart showing an outline of a process of a halftonemodule, which is performed in the image forming apparatus of FIG. 7.

FIGS. 9A and 9B are explanatory views for explaining jetted positions ofcolorless ink in the case of setting a block formed of 2×2 pixels as aunit area and the case of setting a block formed of 4×4 pixels as theunit area.

FIG. 10 is a flowchart showing an outline of a process of a halftonemodule, which is performed in an image forming apparatus of a secondembodiment.

FIGS. 11A, 11B and 11C are specific examples of jetted positions ofrecording ink and the colorless ink in the case of setting a sum totalof adhered amounts of recording ink and colorless ink at 25%.

FIGS. 12A and 12B are specific examples of the jetted positions of therecording ink and the colorless ink in the case of setting the sum totalof the adhered amounts of colorless ink and recording ink at 25%.

FIGS. 13A and 13B are specific examples of the jetted positions of therecording ink and the colorless ink in the case of setting the sum totalof the adhered amounts of colorless ink and recording ink at 25%.

FIGS. 14A, 14B and 14C are specific examples of jetted positions of therecording ink and the colorless ink in the case of setting the sum totalof the adhered amounts of recording ink and colorless ink at 50%.

FIG. 15 is an explanatory view showing an assignment example of therecording ink for each gradation level in Example 1.

FIG. 16 is a graph in which an ink amount of a patch of each gradationlevel in Example 1 is plotted for each ink.

FIGS. 17A, 17B and 17C are explanatory views showing jetted positions ofthe colorless ink for each total sum amount when the sum total of theadhered amounts in Example 1 is changed.

FIG. 18 is a graph showing measurement values of gloss of 25 to 100%patches in Example 1.

FIG. 19 is a graph in which an ink amount of a patch of each gradationlevel in Example 2 is plotted.

FIG. 20 is a graph showing 60-degree gloss values when the colorless inkin Example 2 is changed from 25 to 100%.

FIGS. 21A and 21B are explanatory views for explaining jetted positionsof the colorless ink in Example 3.

BEST MODE FOR CARRYING OUT THE INVENTION First Embodiment

A first embodiment of the present invention will be described below withreference to FIG. 1 to FIG. 9.

In an inkjet recording method of the present invention, like acommercially available inkjet printer, one including a recording mediumhousing unit, a conveyor unit, an ink cartridge, and a recording head ofan inkjet mode can be used without any limitation imposed thereon.However, if the inkjet printer is one of a series of printer sets eachof which comprises at least a roll-like recording medium housing unit,the conveyor unit, the recording head of the inkjet mode, and a cuttingunit, and according to need, a heating unit, a pressurization unit, anda recorded print housing unit, the inkjet printer is useful in the caseof commercially utilizing an inkjet photograph.

First, the inkjet printer to which the inkjet recording method of thepresent invention can be applied will be described with reference toFIG. 1. FIG. 1 is a perspective view showing main constituents of theinkjet printer.

As shown in FIG. 1, in an inkjet printer 1, an image forming unit 2 forjetting ink onto a recording medium and forming an image thereon isprovided. In the image forming unit 2, a platen 21 for supporting, on anupper surface thereof, a back surface (surface opposite to a recordingsurface) of a recording medium P (refer to FIG. 4) in a predeterminedrange is disposed substantially horizontally. Moreover, in the imageforming unit 2, a guide member 25 extended along a scanning direction Xabove the platen 21 and for moving a carriage 23 scanning in thescanning direction X is provided.

On the carriage 23, mounted are recording heads 22 for jetting ink ontothe recording medium, and a linear encoder sensor 27 extended along thescanning direction X and for reading optical patterns on a linear scale26 in which the optical patters are arranged in a cycle of 180 dpi in alongitudinal direction thereof and for outputting the read opticalpatterns as a clock signal. A moving direction of the carriage 23 ischanged in accordance with a rotation direction of a carriage drivemotor 231, and the carriage 23 moves reciprocally in the scanningdirection X. Moreover, at a time of image formation, the carriage 23moves forward, backward or reciprocally in the scanning direction Xwhile the recording medium P is being stopped. Moving speed at this timeis, for example, 705 mm/s at the maximum.

Next, each recording head 22 will be described with reference to FIG. 2and FIG. 3. FIG. 2 is an enlarged perspective view of the carriage 24,and FIG. 3 is a lower surface view of the recording head 22.

The recording head 22 may be any one of a piezoelectric mode, a thermalmode and a continuous mode. However, the piezoelectric mode ispreferable from a viewpoint of stability thereof in pigment ink, and inthis embodiment, the recording head 22 of the piezoelectric mode isused. The recording head 22 is disposed so that the recording surface ofthe recording medium P conveyed on the platen 21 and a nozzle surface222 on which nozzles 221 of the recording head 22 is formed can beopposed to each other.

As shown in FIG. 3, on the nozzle surface 222 of the recording head,nozzle arrays in which 255 pieces of nozzles 221 are formed in line ofapproximately three arrays at a pitch of 141 μm (180 dpi) in theconveying direction are arranged to be shifted from one another by 23.5μm. This is equivalent to one pixel in 1080 dpi. This is for the purposeof, as a drive mechanism of the recording head 22, ensuring that thenozzle 221 drivable simultaneously is every three nozzles. Eachrecording head 22 has a jetting section (not shown) such aspiezoelectric elements provided therein, and individually jets the inkas droplets from the respective nozzles 211 by an operation of thejetting section.

To each recording head 22, the ink is supplied from unillustratedrecording ink cartridges and colorless ink cartridge through pipingtubes. Eight recording heads 22 are arranged in line along the scanningdirection, and are used for dark and light CMK, six in total, Y, and thecolorless ink. In this embodiment, seven kinds of inks which are C, M,Y, K, LC, LM and LK are used as the recording inks. However, even in aninkjet printer for performing the recording by using only the dark C, M,Y and K without using the light colors, an effect of the presentinvention is similar.

Next, a fixing unit 4 for fixing the ink to the recording medium P onwhich the image is formed by the image forming unit 2 will be describedwith reference to FIG. 4. FIG. 4 is a front view showing mainconstituents of the fixing unit 4.

As shown in FIG. 4, the fixing unit 4 is disposed on a downstream sideof the image forming unit 2 in the conveying direction of the recordingmedium P. In the fixing unit 4, there is provided a conveyor roller 42extended in a direction perpendicular to the conveying direction of therecording medium P and for supporting and conveying the recording mediumP from a lower side thereof. To an upper side of the conveyor roller 42,a heating roller 41 formed of a hollow roller faces. In the inside ofthe heating roller 41, a heat source 43 such as a halogen lump heater, aceramic heater and a Nichrome wire is provided. The heating roller 41 isheated by heat of the heat source 43, thermoplastic resin particlescontained in an ink receiving layer of the recording medium P aremelted. A temperature sensor 413 (refer to FIG. 5) is built in theheating roller 41. Moreover, a gear 412 is formed on a peripheral edgeof an end of the heating roller 41, and meshes with a gear 441 attachedonto a heating roller drive motor 44. By these gear 441 and gear 441,drive force of the heating roller drive motor 44 is adapted to betransmitted to the heating roller 41, and to rotationally drive theheating roller 41 in a predetermined direction. It is preferable thatthe heating roller 41 be formed of a material high in thermalconductivity so as to make it possible to heat the recording medium Pefficiently by the heat radiated from the heat source 43. For example, ametal roller is mentioned. It is preferable that, on the surface of theheating roller 41, fluorine resin be coated in order to preventcontamination owing to the ink at the time of heating and pressurizingthe recording medium P. Besides, a silicon rubber roller coated withheat-resistant silicon rubber can also be used.

Next, a control circuit of the inkjet printer 1 will be described withreference to FIG. 5. FIG. 5 is a block diagram showing the controlcircuit of the inkjet printer 1.

As shown in FIG. 5, a control circuit 100 is configured in such a mannerthat a conveyor motor 101 for conveying the recording medium P, arecording medium type determination sensor 102 for determining a type ofthe recording medium P, a CPU 103, an interface 104, the carriage drivemotor 231, the heating roller drive motor 44, the temperature sensor413, the heat source 43, a memory write controller 105, an image memory106, a memory read controller 107, a head driver 108, and the recordinghead 22, are connected to one another through a bus 110. Note that,besides these, the respective drive units of the inkjet printer 1, andthe like are connected to the control circuit 100.

The control circuit 100 controls conveyance of the recording medium P, ascanning operation of the carriage 23, the ink jetting of the recordinghead 22, and the like.

When the control unit 100 controls the recording head 22, as shown inFIG. 6, data of 255 pixels in each array of the respective recordingheads 22 is read for a time of three cycles of a pixel clock as a cycleof 33 μs, that is, for 100 μs, and is transferred to the head driver108. The head driver 108 generates a head drive pulse signal respondingto the data of three values corresponding to each nozzle 221 at timingcorresponding to a phase of each nozzle 221. Specifically, the headdriver 108 does not generate the pulse signal when the data is “0”,generates one pulse when the data is “1”, and generates two pulses at aninterval of approximately 10 μs when the data is “2”. Moreover, headdrive pulses in the respective phases A, B and C are generated at timingshifted by 33 μs for one pixel clock.

Moreover, as shown in FIG. 5 and FIG. 7, an image forming apparatus 200such as a computer is connected to the control circuit 100. The imageforming apparatus 200 forms a multicolor image based on a signalinputted thereto. In this example, an application program 201 operatingin the inside of the image forming apparatus 200 displays the image on amonitor 300 through a video driver 202 while performing a process forthe image. When the application program 201 issues an instruction toform the image, a printer driver 203 of the image forming apparatus 200receives image data for forming the image from the application program201, and converts the image data into a signal by which the image isformable in the inkjet printer 1. Specifically, in this embodiment, thecontrol circuit 100 and the image forming apparatus 200 will serve as acontrol unit of the present invention.

The printer driver 203 includes a rasterizer 204 for converting theimage data handled by the application program 201 into color informationper dot unit, a color gradation correction module 205 for correcting theimage gradation data converted into the color information per dot unitin accordance with color reproduction property and gradation property ofthe inkjet printer 1, a halftone module 206 for generating-so-calledhalftone image data expressing a density on a certain area, that is,data for the recording ink, which expresses the jetted position, theadhered amount and the like of the recording ink, by the presence orabsence of the recording ink per dot unit from the image data havingbeen subjected to color correction, and a colorless ink calculationmodule 207 for generating data for the colorless ink, which representsthe jetted position and adhered amount of the colorless ink based on thedata for the recording ink, which is generated in the halftone module206.

Next, the recording medium for use in this embodiment will be described.

Mediums in general for use in usual inkjet recording are applicable asthe recording medium; however, a type having an ink absorbing layer on asupport is preferable from a viewpoint of the image quality, and thetype includes a swelling type and a micro-porous type.

As the one of the swelling type, usable is one formed by applying, forexample, gelatin, polyvinyl alcohol, polyvinyl pyrrolidone, polyethyleneoxide and the like alone or in combination as a hydrophilic binder, andby making the hydrophilic binder as the ink absorbing layer.

In the recording medium having the micro-porous ink absorbing layer, thegloss is significantly improved by the recording ink, and accordingly,the recording medium serves as a preferred embodiment of the presentinvention. The ink absorbing layer of the micro-porous recording mediummay be configured to have either a single layer, or two or more layers.In particular, it is preferable to use an inkjet recording medium havingan ink absorbing layer of a double-layer configuration, in which a firstink absorbing layer containing inorganic pigment is provided on asupport, and thereon, a second ink absorbing layer containingthermoplastic resin to be described later and inorganic pigment areprovided. The micro-porous ink absorbing layer will be descried belowmore in detail.

(Micro-Porous Ink Absorbing Layer)

A micro-porous layer is one mainly formed by soft aggregation of thehydrophilic binder and the inorganic pigment. Heretofore, variousmethods for forming micropores in a coating film have been known, forexample, which are: a method of applying a uniform coating solutioncontaining two or more polymers onto a support, and allowing thesepolymers to cause phase separation in a drying process, thereby formingmicropores; a method of applying a coating solution containing solidfine particles and a hydrophilic or hydrophobic binder onto a support,and after drying, immersing an inkjet recording medium in water or aliquid containing an appropriate organic solvent to dissolve the solidfine particles, thereby forming the micropores; a method of applying acoating solution containing a compound having property to foam at thetime of forming the coating film, and thereafter, foaming the compoundin a drying process, thereby forming the micropores in the coating film;a method of applying a coating solution containing porous solid fineparticles and a hydrophilic binder onto a support, thereby forming themicropores in the porous solid fine particles and between the fineparticles; a method of applying, onto a support, a coating solutioncontaining solid fine particles and/or fine particle oil droplets havinga volume nearly equal to or more than that of a hydrophilic binder andthe hydrophilic binder, thereby forming the micropores between the solidfine particles; and the like. In the present invention, it isparticularly preferable that the micropores be formed by allowing themicro-porous layer to contain various inorganic solid fine particleswith a mean particle diameter of 100 nm or less.

As the inorganic pigment for use under the above-described object, forexample, white inorganic pigment and the like are mentioned, such assoft calcium carbonate, heavy calcium carbonate, magnesium carbonate,kaolin, clay, talc, calcium sulfate, barium sulfate, titanium dioxide,zinc oxide, zinc hydroxide, zinc sulfide, zinc carbonate, hydrotalcite,aluminium silicate, diatomite, calcium silicate, magnesium silicate,synthetic amorphous silica, colloidal silica, alumina, colloidalalumina, pseudoboehmite, aluminium hydroxide, lithopone, zeolite andmagnesium hydroxide.

The mean particle diameter of the inorganic pigment is obtained as asimple mean value (number mean) by observing the particles or particlesappearing on a cross section or a surface of the micro-porous layer bymeans of an electron microscope and by randomly measuring particlediameters of 1,000 particles. Here, the particle diameter of eachparticle is one represented by a diameter when a circle equal to aprojection area thereof is assumed. As the solid fine particles, it ispreferable to use solid fine particles of a substance selected fromsilica, alumina and alumina hydrate, and more preferable to use solidfine particles of silica.

As the silica, preferably used is silica synthesized by a usual wetmethod, colloidal silica, silica synthesized by a vapor phase method, orthe like. Fine particle silica preferably used in particular in thepresent invention is the colloidal silica or fine particle silicasynthesized by the vapor phase method. Among them, the fine particlesilica synthesized by the vapor phase method is preferable because notonly the fine particle silica can obtain a high void rate but also isless prone to form a rough and large aggregation when being added to acationic polymer for the purpose of immobilizing dye. Moreover, aluminaor alumina hydrate may be either crystalline or amorphous, and it ispossible to use those with any shape, such as undefined shapedparticles, spherical particles and needle particles.

With regard to the fine particles, it is preferable that a fine particledispersion thereof before being mixed with the cationic polymer be in astate of being dispersed into primary particles.

It is preferable that a particle diameter of the inorganic pigment be100 nm or less. For example, in the case of the above-descried fineparticle silica by the vapor phase method, a mean particle diameter(particle diameter in a dispersion state before application thereof) ofthe primary particles of the inorganic pigment dispersed in a state ofthe primary particles concerned is, preferably 100 nm or less, morepreferably 4 to 50 nm, most preferably 4 to 20 nm.

As the most preferably used silica synthesized by the vapor phasemethod, in which the mean particle diameter of the primary particles is4 to 20 nm, for example, Aerosil of Nippon Aerosil Co., Ltd. iscommercially available. Such fine particle silica by the vapor phasemethod can be dispersed into the primary particles relatively easily,for example, by being sucked and dispersed by a jet stream inductormixer made by Mitamura Riken Kogyo Inc.

The hydrophilic binder includes, for example, polyvinyl alcohol,gelatin, polyethylene oxide, polyvinyl pyrrolidone, polyacrylic acid,polyacrylamide, polyurethane, dextran, dextrin, carrageenan (κ, l, λ,etc.), agar, pullulan, water-soluble polyvinyl butyral,hydroxyethylcellulose, carboxymethylcellulose, and the like. It ispossible to combine two or more of these water-soluble resins.

The water-soluble resin preferably used in the present invention ispolyvinyl alcohol. In the polyvinyl alcohol preferably used in thepresent invention, modified polyvinyl alcohols such as polyvinyl alcoholwhere a terminal thereof is modified with cation and anion modifiedpolyvinyl alcohol having an anionic group are included besides usualpolyvinyl alcohol obtained by hydrolyzing polyvinyl acetate.

As polyvinyl alcohol obtained by hydrolyzing vinyl acetate, those withan average polymerization degree of 1,000 or more are preferably used,and particularly those with an average polymerization degree of 1,500 to5,000 are preferably used. A saponification degree is preferably from 70to 100%, and particularly preferably 80 to 99.5%.

The cation modified polyvinyl alcohol is, for example, the polyvinylalcohol having primary to tertiary amino groups and quaternary ammoniumgroups in a backbone or side chains as described inJP-Tokukaisho-61-10483A, and this is obtained by saponifying a copolymerof an ethylenic unsaturated monomer having cationic group and vinylacetate.

As the ethylenic unsaturated monomer having cationic group, for example,trimethyl-(2-acrylamide-2,2-dimethylethyl)ammonium chloride,trimethyl-(3-acrylamide-3,3-dimethylpropyl)ammonium chloride,N-vinylimidazole, N-vinyl-2-methylimidazole,N-(3-dimethylaminopropyl)methacrylamide, hydroxylethyltrimethylammoniumchloride, trimethyl-(2-methacrylamidepropyl)ammonium chloride,N-(1,1-dimethyl-3-dimethylaminopropyl)acrylamide, and the like, areincluded.

A percent of cation modified group-containing monomer of the cationmodified polyvinyl alcohol is from 0.1 to 10 mol %, and preferably from0.2 to 5 mol % based on vinyl acetate.

As anion modified polyvinyl alcohol, for example, polyvinyl alcoholhaving an anionic group as described in JP-Tokukaihei-1-206088A,copolymers of vinyl alcohol and a vinyl compound having a water-solublegroup as described in JP-Tokukaisho-61-237681A andJP-Tokukaisho-63-3079799A, and modified polyvinyl alcohol having awater-soluble group as described in JP-Tokukaihei-7-285265A areincluded.

As nonionic modified polyvinyl alcohol, for example, polyvinyl alcoholderivatives where a polyalkyleneoxide group is added to a part of vinylalcohol as described in JP-Tokukaihei-7-9758A, a block copolymer of avinyl compound having a hydrophobic group and vinyl alcohol as describedin JP-Tokukaihei-8-25795A, and the like are included.

Polyvinyl alcohol can be also used in combination with two or moredepending on difference in polymerization degree and modification type.

An added amount of the inorganic pigment for use in the ink absorbinglayer largely depends on a required ink absorbing capacity, a void rateof the micro-porous layer, a type of the inorganic pigment and a type ofthe water-soluble resin; however, the added amount is 5 to 30 g inusual, and preferably 10 to 25 g per m² of a recording sheet.

Moreover, a ratio of the inorganic pigment and the water-soluble resinwhich are for use in the ink absorbing layer is 2:1 to 20:1 in usual,and preferably 3:1 to 10:1 in a mass ratio.

The ink absorbing layer may also contain a cationic water-solublepolymer having quaternary ammonium bases in molecules, and the cationicwater-soluble polymer is used in a range of 0.1 to 10 g in usual, andpreferably 0.2 to 5 g per m² of the inkjet recording medium.

In the micro-porous layer, it is preferable that a total amount (voidcapacity) of the micropores be 20 ml or more per m² of the recordingsheet. In the case where the void capacity is less than 20 ml/m², thoughink absorptivity is good when an ink amount at the time of printing issmall, the ink is not completely absorbed when the ink amount isincreased, making it prone to cause such problems as lowering of theimage quality and a delay in drying property.

In the micro-porous layer having a function to retain the ink, the voidcapacity with respect to a solid capacity is referred to as a void rate.In the present invention, it is preferable to set the void rate at 50%or more because it is possible to form the micropores efficientlywithout unnecessarily thickening a film thickness of the micro-porouslayer.

With regard to other types of the micro-porous ink absorbing layer, suchan ink solvent absorbing layer may be formed by using a coating solutionprepared by combining a polyurethane resin emulsion and a water-solubleepoxy compound and/or acetoacetylated polyvinyl alcohol with each other,and further, combining epichlorohydrin polyamide resin therewith,besides forming the ink solvent absorbing layer by using the inorganicpigment. As the polyurethane resin emulsion in this case, preferable isa polyurethane resin emulsion in which a diameter of particles havingpolycarbonate chains, polycarbonate chains and polyester chains is 3.0μm. It is more preferable that polyurethane resin of the polyurethaneresin emulsion obtained by reacting an aliphatic isocyanate compound andpolyol having polycarbonate polyol, polycarbonate polyol and polyesterpolyol with each other have sulfonic acid groups in molecules, andfurther, have epichlorohydrin polyamide resin and a water-soluble epoxycompound and/or acetoacetylated vinyl alcohol.

It is assumed that, in the ink solvent absorbing layer using theabove-described polyurethane resin, a weak aggregation of the cation andthe anion is formed, followed by forming of micropores having power toabsorb the ink solvent, thereby enabling the image formation.

(Thermoplastic Resin-Containing Layer)

In the present invention, a layer containing the thermoplastic resin canbe provided on the surface layer of the ink absorbing layer. The layercontaining the thermoplastic resin may be either a layer formed only ofthe thermoplastic resin or one added with a water-soluble binderaccording to needs. However, one is preferable, to which both of thewater-soluble binder and the inorganic pigment are added. As theinorganic pigment addable to the thermoplastic resin, usable is thematter previously described in the explanation of the ink absorbinglayer.

It is preferable that the thermoplastic resin be formed into a fineparticle shape from a viewpoint of ink permeability. The thermoplasticresin or the fine particles thereof include, for example, polycarbonate,polyacrylonitrile, polystyrene, polyacrylic acid, polymethacrylic acid,an acrylic ester copolymer, polyvinyl chloride, polyvinylidene chloride,polyvinyl acetate, polyester, polyamide, polyether, copolymers thereof,and salts thereof. Among them, preferable are a styrene-acrylic acidester copolymer, a methacrylic acid ester-acrylic acid ester copolymer,a vinyl chloride-vinyl acetate copolymer, an acrylic ester copolymer, avinyl chloride-acrylic acid ester copolymer, an ethylene-vinyl acetatecopolymer, an ethylene-acrylic acid ester copolymer, and SBR latex. Morepreferable thermoplastic resin is the acrylic ester copolymer.

For the thermoplastic resin or the fine particles thereof, pluralpolymers different in monomer composition, particle diameter andpolymerization degree may be mixed together for use.

When selecting the thermoplastic resin or the fine particles thereof,ink receiving property, the gloss of the image after the fixing by theheating and the pressurization, image robustness and mold releaseproperty should be considered.

With regard to the ink receiving property, when the particle diameter ofthe thermoplastic fine particles is less than 0.05 μm, separation ofpigment particles in the pigment ink and the ink solvent becomes slow,bringing lowering of the ink absorption speed. Meanwhile, when theparticle diameter exceeds 10 μm, this is not preferable from viewpointsof adhesiveness of the thermoplastic resin to a solvent absorbing layeradjacent to the ink absorbing layer when the thermoplastic resin isapplied onto the support, coating film strength of the inkjet recordingmedium after the application and drying thereof, development of thegloss, and the like. Therefore, the particle diameter of thethermoplastic resin is preferably 0.05 to 10 μm, more preferably 0.1 to5 μm, and far more preferably 0.1 to 1 μm.

Moreover, a criterion for selecting the thermoplastic resin or the fineparticles thereof includes a glass transition point (Tg). When Tg islower than application and drying temperature, for example, theapplication and drying temperature at the time of preparing therecording medium is already higher than Tg, the micropores by thethermoplastic fine particles for transmitting the ink solventtherethrough will disappear.

Moreover, when Tg is equal to or more than temperature at whichdenaturation owing to the heat of the support occurs, a fixing operationat high temperature becomes necessary because the pigment ink is meltedand deposited after the inkjet recording thereby, and a load on theapparatus, thermal stability of the support, and the like will becomeproblems. Preferable Tg of the thermoplastic fine particles is 50 to150° C. Moreover, 50 to 150° C. is preferable as the minimum filmforming temperature (MFT).

From a viewpoint of environmental adaptability, as the fine particles ofthe thermoplastic resin, one dispersed in an aqueous system ispreferable, and in particular, aqueous latex obtained by emulsionpolymerization is preferable. In this case, a type subjected to theemulsion polymerization by using a nonionic dispersant as an emulsifieris preferably usable. Moreover, it is preferable that the fine particlesof the thermoplastic resin have a less remaining monomer component fromviewpoints of an odor and safety, and the remaining monomer componentwith respect to a solid mass of the polymer is preferably 3% or less,more preferably 1% or less, and particularly preferably 0.1% or less.Moreover, it is preferable that a remaining polymerization initiator bea little. While it is preferable that a ratio of the remainingpolymerization initiator be 0.5% or less with respect to the solid massof the polymer, it is the most preferable that the polymerizationinitiator should not remain at all.

As the water-soluble binder, polyvinyl alcohol and polyvinyl pyrrolidoneare usable within a range of 1 to 10% of the fine particles of thethermoplastic resin.

It is preferable that the recording medium include the ink absorbinglayer on the support, and that the surface layer thereof contain atleast the inorganic pigment and the fine particles of the thermoplasticresin. In particular, the following points can be mentioned as reasonsthat it is preferable as above.

1) The ink absorption speed is large, image deterioration such asbeading and color bleed is less prone to occur, and adaptability tohigh-speed printing is inherent.

2) Strength of the image surface is strong.

3) Sticking when the images are stacked on one another at the time ofstorage is less prone to occur.

4) Application productivity of the ink absorbing layer is excellent.

5) Writability is provided.

In this case, it is preferable to determine solid mass ratios of thefine particles of the thermoplastic resin and the inorganic pigment onthe surface layer individually in accordance with the fine particles ofthe thermoplastic resin, the inorganic pigment, other additives and thelike, and no particular limitations are imposed thereon. However, theratio of the fine particles of the thermoplastic resin and the inorganicpigment is preferably 2/8 to 8/2, more preferably, 3/7 to 7/3, and farmore preferably 4/6 to 6/4.

(Support)

As the support, usable is a support used heretofore for the inkjetrecording medium, for example, a paper support such as regular paper,art paper, coated paper and cast-coated paper, a plastic support, apaper support having both surfaces coated with polyolefin, and a complexsupport formed by pasting these together.

For the purpose of increasing adhesion strength of the support and theink receiving layer, it is preferable to give a corona dischargetreatment, an undercoating treatment and the like to the support priorto the application of the ink receiving layer. Moreover, it is notalways necessary that the recording medium be invisible, and therecording medium may also be colored. Moreover, it is particularlypreferable to use a paper support in which both surfaces of an originalsupport are laminated with polyethylene because a recorded imageapproaches photograph image quality and a high-quality image can beobtained at low cost.

The paper support as described above, which is laminated withpolyethylene, will be described below. Original paper for use in thepaper support is formed of wood pulp as a main material, and accordingto needs, synthetic pulp such as polypropylene or synthetic fiber suchas nylon and polyester is used in addition to the wood pulp, and theoriginal paper is thus formed into paper. As the wood pulp, for example,any of LBKP, LBSP, NBKP, NBSP, LDP, NDP, LUKP and NUKP can be used.However, it is preferable to use more LBKP, NBSP, LBSP, NDP and LDP eachcontaining many short fibers. Note that a ratio of the LBSP or the LDPis preferably 10 to 70 mass %.

For the above-described pulp, chemical pulp (sulfate pulp and sulfitepulp) with little impurities is preferably used, and moreover, pulp inwhich a whiteness degree is improved by undergoing a bleaching treatmentis also useful.

Into the original paper, for example, a sizing agent such as higherfatty acid and alkylketene dimer, white pigment such as calciumcarbonate, talc and titanium oxide, a paper power enhancer such asstarch, polyacrylamide and polyvinyl alcohol, a fluorescent brighteningagent, a moisture retaining agent such as polyethylene glycols, adispersant, a softening agent such as quaternary ammonium, and the like,can be added as appropriate.

A freeness of the pulp for use in making the paper is preferably 200 to500 ml in the definition of the CSF, and the sum of a mass percent of 24mesh residue and a mass percent of 42 mesh residue, in which fiberlength after being beaten is defined in JIS-P-8207, is preferably 30 to70%. Note that it is preferable that a mass percent of 4 mesh residue be20 mass % or less.

Basis weight of the original paper is preferably 30 to 250 g,particularly preferably 50 to 200 g. Thickness of the original paper ispreferably 40 to 250 μm.

The original paper undergoes a calendering treatment at a paper-makingstep or after the paper making, thus also making it possible to imparthigh smoothness thereto. In general, a density of the original paper is0.7 to 1.2 g/m² (JIS-P-8118). Moreover, a stiffness of the originalpaper is preferably 20 to 200 g under the condition defined inJIS-P-8143.

A surface sizing agent may be applied onto the surface of the originalpaper. As the surface sizing agent, the sizing agent such as higherfatty acid and alkylketene dimer, which is addable into theabove-described original paper, is usable.

pH of the original paper is preferably 5 to 9 when being measured by ahot water extraction method defined in JIS-P-8113.

Polyethylene that coats the surface and back surface of the originalpaper is mainly low-density polyethylene (LDPE) and/or high-densitypolyethylene (HDPE); however, LLDPE, polypropylene and the like otherthan the above are also usable partially.

In particular, for the polyethylene layer on the ink absorbing layerside, one is preferable, in which, like being widely performed inphotographic printing paper, rutile or anatase-type titanium oxide isadded into the polyethylene, thereby improving opacity and the whitenessdegree. A content of the titanium oxide is 3 to 20 mass % in usual, andpreferably 4 to 13 mass % with respect to the polyethylene.

The polyethylene-coated paper is usable as glossy paper. Moreover, oneon which a matte surface as obtained in a usual photographic printingpaper is formed by performing a so-called embossing process when thepolyethylene is melted and extruded onto the surface of the originalpaper and coated thereon is also usable in the present invention.

A usage amount of the polyethylene on the front and back of the originalpaper is selected so as to optimize curls under low humidity and highhumidity after the micro-porous layer and a backing layer are provided.However, in usual, in terms of thickness, a polyethylene layer on themicro-porous layer side is within a range of 20 to 40 μm, and apolyethylene layer on the backing layer side is within 10 to 30 μm.

[Preparation of Recording Medium]

A preparation method of the recording medium for use in this embodimentwill be described by taking a specific example.

On the paper support (having thickness of 220 μm and containing 13 mass% of the anatase-type titanium oxide with respect to the polyethylene ofthe ink absorbing layer surface in the polyethylene) in which bothsurfaces were coated with the polyethylene, an underlayer coatingsolution to be described later was applied as a first layer from thesupport side, and a surface layer coating solution to be described laterwas simultaneously applied thereon as a second layer by a slide hopper,followed by drying, thereby preparing the recording medium 1.

Note that the coating solution was applied while being heated up to 40°C., and immediately after the application, the recording medium 1 wascooled down for 20 seconds in a cooling zone maintained at 0° C.Thereafter, the recording medium 1 was sequentially dried for 60 secondsin a wind (relative humidity: 15%) of 25° C., for 60 seconds in a wind(relative humidity: 25%) of 45° C., and for 60 seconds in a wind(relative humidity: 25%) of 50° C., and was conditioned in humidity for2 minutes under the atmosphere where the relative humidity is 40 to 60°C. Then, a sample was taken up. Note that the application was performedso that an attached amount of silica could be 18 g/m² in the underlayer,and that an attached amount of silica could be 3 g/m² in the surfacelayer.

To the above-described coating solution, UVITE NFW LIQUID (prepared byCiba Specialty Chemicals Inc.) as a water-soluble fluorescentbrightening agent was added to reach an amount of 100 mg/m². Moreover,to the above-described coating solution, the same fluorescentbrightening agent was added to reach an amount of 20 mg/m².

(Preparation of Silica Dispersion)

125 kg of vapor phase method silica (QS-20: prepared by Tokuyama Corp.)in which a mean particle diameter of primary particles was approximately0.012 μm was sucked and dispersed at room temperature in 620 L of purewater of which pH was adjusted at 2.5 by nitric acid by using the jetstream inductor mixer TDS made by Mitamura Riken Kogyo Inc.Subsequently, a silica dispersion was finished into a total amount of694 L by pure water.

Next, to 18 L of a solution (ph=2.3) containing 1.14 kg of cationicpolymer P-1, 2.2 L of ethanol and 1.5 L of n-propanol, 69.4 L of theabove-described silica dispersion was added while being agitated.Subsequently, 7.0 L of an aqueous solution containing 260 g of boricacid and 230 g of borax was added, and 1 g of an antifoaming agent SN381(prepared by San Nopco Limited) was added. This mixed solution wasdispersed by a high-pressure homogenizer made by Sanwa Industries Co.,Ltd., and was finished into a total amount of 97 L by pure water,thereby preparing the silica dispersion.

(Preparation of Underlayer Coating Solution)

While 600 ml of the above-described silica dispersion was being agitatedat 40° C., the respective additives to be described below weresequentially mixed therewith, thereby preparing the underlayer coatingsolution. 10% aqueous solution of polyvinyl alcohol  6 ml (PVA203:prepared by Kuraray Co., Ltd.) 7% aqueous solution of polyvinyl alcohol185 ml (PVA235: prepared by Kuraray Co., Ltd.) Saponin (50% aqueoussolution) appropriate amount Pure water equivalent to finish the totalamount to 1000 ml

(Preparation of Surface Layer Coating Solution)

After being prepared, the above-described underlayer coating solutionwas agitated at 43° C. for 30 minutes, and subsequently, thermoplasticfine particles (acrylic latex, 82° C. of Tg, 160 nm of number meanparticle diameter, 25% of solid content) were added thereto during 15minutes so that a solid content ratio of the thermoplastic fineparticles/filler (silica) could be 55/45, thereby preparing the surfacelayer coating solution 1. After being filtered by a filter of 10 μm thesurface layer coating solution 1 was used for the application.

Next, the recording ink and the colorless ink for use in the inkjetprinter 1 of this embodiment will be described.

[Recording Ink]

The color material of the recording ink may be either the dye or thepigment as long as the recording ink has adaptability to the inkjet modein general. Particularly, in the case of making much of the viewpointsof the image permanence and the image quality, the pigment ink ispreferable.

(Pigment)

As the pigment, organic pigment such as insoluble pigment and lakepigment and carbon black are preferably usable. No particularlimitations are imposed on the insoluble pigment. However, for example,azo, azomethine, methane, diphenylmethane, triphenylmethane,quinacridone, anthraquinone, perylene, indigo, quinophthalone,isoindolinone, isoindoline, azine, oxazine, thiazine, dioxazine,thiazole, phthalocyanine, diketopyrrolopyrrole, and the like arepreferable. As specific pigment preferably usable, the followingpigments can be given.

As pigment for magenta or red, for example, C.I. Pigment Red 2, C.I.Pigment Red 3, C.I. Pigment Red 5, C.I. Pigment Red 6, C.I. Pigment Red7, C.I. Pigment Red 15, C.I. Pigment Red 16, C.I. Pigment Red 48:1, C.I.Pigment Red 53:1, C.I. Pigment Red 57:1, C.I. Pigment Red 122, C.I.Pigment Red 123, C.I. Pigment Red 139, C.I. Pigment Red 144, C.I.Pigment Red 149, C.I. Pigment Red 166, C.I. Pigment Red 177, C.I.Pigment Red 178, C.I. Pigment Red 222 and the like are given.

As pigment for orange or yellow, for example, C.I. Pigment Orange 31,C.I. Pigment Orange 43, C.I. Pigment Yellow 12, C.I. Pigment Yellow 13,C.I. Pigment Yellow 14, C.I. Pigment Yellow 15, C.I. Pigment Yellow 17,C.I. Pigment Yellow 93, C.I. Pigment Yellow 94, C.I. Pigment Yellow 138and the like are given.

As pigment for green or cyan, for example, C.I. Pigment Blue 15, C.I.Pigment Blue 15:2, C.I. Pigment Blue 15:3, C.I. Pigment Blue 16, C.I.Pigment Blue 60, C.I. Pigment Green 7 and the like are given.

For these pigments, a pigment dispersion may be used. As a usablepigment dispersion, for example, a surfactant such as higher fatty acidsalt, alkyl sulfate salt, alkyl ester sulfate salt, alkyl sulfonatesalt, sulfosuccinate salt, naphthalene sulfonate salt, alkyl phosphatesalt, polyoxyalkylenealkylether phosphate salt,polyoxyalkylenealkylphenyl ether, polyoxyethylenepolyoxypropyleneglycol, glycerin ester, sorbitan ester, polyoxyethylenefatty acid amide and amine oxide, or a block copolymer and a randomcopolymer each of which is made up of two or more monomers selected fromstyrene, a styrene derivative, a vinylnaphthalene derivative, acrylicacid, an acrylate derivative, maleic acid, a maleate derivative,itaconic acid, an itaconate derivative, fumaric acid and a fumaratederivative, and salts thereof can be given.

With regard to a dispersion method of the pigment, means thereof is notparticularly limited. However, for example, a ball mill, a sand mill, anattritor, a roll mill, an agitator, a Henschel mixer, a colloid mill, anultrasonic homogenizer, a pearl mill, a wet type jet mill, a paintshaker and the like are usable.

To use a centrifuging machine and to use a filter are also preferredmethods for the purpose of eliminating coarse particles of a pigmentdispersion according to the present invention.

A mean particle diameter of the pigment in the pigment ink is selectedin consideration of the stability, image density, glossiness, lightresistance thereof in the ink. In addition, in the inkjet recordingmethod of the present invention, it is preferable to select the particlediameter also from viewpoints of improving the gloss and improving thetexture. Although reasons that the selection of the particle diameterpromotes the improvement of the gloss and the improvement of the texturein the present invention are uncertain, it is assumed that the reasonsare associated with the fact that the pigment in the image is in a stateof being dispersed into the coating film where the fine particles of thethermoplastic resin are melted. In the case of aiming at high-speedprocessing, the fine particles of the thermoplastic resin must be meltedand formed into the coating film in a short time, and further, thepigment must be dispersed into the coating film sufficiently. At thistime, the surface area of the pigment has a great influence, andtherefore, it is assumed that the mean-particle diameter has the optimumrange.

(Water-Soluble Organic Solvent)

It is preferable that an aqueous ink composition as a preferredembodiment as the pigment ink be combined with a water-soluble organicsolvent.

As the water-soluble organic solvent, for example, mentioned can bealcohols (for example, methanol, ethanol, propanol, isopropanol,butanol, isobutanol, secondary butanol, tertiary butanol, pentanol,hexanol, cyclohexanol, benzyl alcohol, and the like), polyvalentalcohols (for example, ethyleneglycol, diethyleneglycol,triethyleneglycol, polyethyleneglycol, propyleneglycol,dipropyleneglycol, polypropyleneglycol, butyleneglycol, hexanediol,pentanediol, glycerin, hexanetriol, thiodiglycol, and the like),polyvalent alcohol ethers (for example, ethyleneglycol monomethylether,ethyleneglycol monoethylether, ethyleneglycol monobutylether,diethyleneglycol monomethylether, diethyleneglycol monomethylether,diethyleneglycol monobutylether, propyleneglycol monomethylether,propyleneglycol monobutylether, ethyleneglycol monomethylether acetate,triethyleneglycol monomethylether, triethyleneglycol monoethylether,triethyleneglycol monobutylether, ethyleneglycol monophenylether,propyleneglycol monophenylether, and the like), amines (for example,ethanolamine, diethanolamine, triethanolamine, N-methyldiethanolamine,N-ethyldiethanolamine, morpholine, N-ethylmorpholine, ethylenediamine,diethylenediamine, triethylenetetramine, tetraethylenepentamine,polyethyleneimine, pentamethyldiethylenetriamine,tetramethylpropylenediamine, and the like), amides (for example,formamide, N,N-dimethylformamide, N,N-dimethylacetamide, and the like),heterocycles (for example, 2-pyrrolidone, N-methyl-2-pyrrolidone,cyclohexylpyrrolidone, 2-oxazolidone, 1,3-dimethyl-2-imidazolidinone,and the like), sulfoxides (for example, dimethylsulfoxide and the like),sulfones (for example, sulforane and the like), urea, acetonitrile,acetone, and the like). The polyvalent alcohols can be given as apreferable water-soluble organic solvent. Moreover, it is particularlypreferable to combine the polyvalent alcohol and the polyvalent alcoholether.

The water-soluble organic solvents may be used alone or in combination.An added amount of the water-soluble organic solvent into the ink is 5to 60 mass % in total, and preferably 10 to 35 mass %.

(Fine Particle of Thermoplastic Resin)

To the ink composition, the fine particles of the thermoplastic resin, aviscosity adjuster, a surface tension adjuster, a specific resistanceadjuster, a film forming agent, a dispersant, a surfactant, anultraviolet ray absorbent, an anti-oxidant, an anti-color fading agent,a mildewproofing agent, an anti-rusting agent and the like areappropriately added for the purpose of improvements of the jettingstability, compatibility of a print head and an ink cartridge, storagestability, image permanence and other various performances.

In particular, it is preferable to add the fine particles of thethermoplastic resin because the gloss of the image is improved. Withregard to the fine particles of the thermoplastic resin, the typesthereof already described in the explanation of the thermoplastic resinaddable onto the surface layer of the recording medium P or the fineparticles thereof are usable. In particular, it is preferable to applyone that does not cause thickening, precipitation and the like even ifbeing added to the recording ink. The mean particle diameter of the fineparticles of the thermoplastic resin is preferably 0.5 μm or less, morepreferably, is selected within a range of 0.2 times to twice the meanparticle diameter of the pigment in the recording ink. Then, this ispreferable in terms of the stability. The fine particles of thethermoplastic resin to be added are preferably ones to be melted andsoftened within a range of 50 to 200° C.

(Ink Composition)

With regard to the ink composition, 40 mPa·s or less is preferable asviscosity thereof at the time of flying, and 30 mPa·s or less is morepreferable. 20 mN/m or more is preferable as surface tension of the inkcomposition at the time of flying, and 30 to 45 mN/m is more preferable.

A solid concentration of the pigment in the recording ink can beselected within a range of 0.1 to 10 mass %. In order to obtain aphotographic image, it is preferable to use so-called dark and lightinks in which concentrations of pigment solid contents are variedindividually, and is particularly preferable to individually use darkand light inks of yellow, magenta, cyan and black. Moreover, it ispreferable to also use special inks of red, green, blue and the likeaccording to needs in terms of the color reproduction.

[Preparation of Pigment Ink Set]

A preparation method of the recording ink for use in this embodimentwill be described below by taking specific examples.

(Preparation of Pigment Dispersion)

<Preparation of Yellow Pigment Dispersion> C.I. Pigment Yellow 74 20mass % Styrene-acrylic acid copolymer (molecular weight of 12 mass %10,000, acid value of 120) Diethyleneglycol 15 mass % Ion-exchange water53 mass %

The above-described respective additives were mixed together, dispersedby using a horizontal type bead mill (System Zeta Mini made by AshizawaFinetech Ltd.) in which zirconia beads of 0.3 mm were filled at a volumefraction of 60%, and a yellow pigment dispersion was thus obtained. Amean particle diameter of the obtained yellow pigment was 112 nm.

<Preparation of Magenta Pigment Dispersion> C.I. Pigment Red 122 25 mass% Joncryl 61 (acrylic-styrene resin prepared by 18 mass % as JohnsonPolymer Corporation) solid content Diethyleneglycol 15 mass %Ion-exchange water 42 mass %

The above-described respective additives were mixed together, dispersedby using the horizontal type bead mill (System Zeta Mini made byAshizawa Finetech Ltd.) in which the zirconia beads of 0.3 mm werefilled at the volume rate of 60%, and a magenta pigment dispersion wasthus obtained. A mean particle diameter of the obtained magenta pigmentwas 105 nm.

<Preparation of Cyan Pigment Dispersion> C.I. Pigment Blue 15:3 25 mass% Joncryl 61 (acrylic-styrene resin prepared by 15 mass % JohnsonPolymer Corporation) as solid content Glycerin 10 mass % Ion-exchangewater 50 mass %

The above-described respective additives were mixed together, dispersedby using the horizontal type bead mill (System Zeta Mini made byAshizawa Finetech Ltd.) in which the zirconia beads of 0.3 mm werefilled at the volume fraction of 60%, and a cyan pigment dispersion wasthus obtained. A mean particle diameter of the obtained cyan pigment was87 nm.

<Preparation of Black Pigment Dispersion> Carbon black 20 mass %Styrene-acrylic acid copolymer 10 mass % (molecular weight of 7,000,acid value of 150) Glycerin 10 mass % Ion-exchange water 60 mass %

The above-described respective additives were mixed together, dispersedby using the horizontal type bead mill (System Zeta Mini made byAshizawa Finetech Ltd.) in which the zirconia beads of 0.3 mm werefilled at the volume fraction of 60%, and a black pigment dispersion wasthus obtained. A mean particle diameter of the obtained black pigmentwas 75 nm.

(Preparation of Pigment Ink Set)

<Preparation of Dark Yellow Ink> Yellow pigment dispersion 15 mass %Ethyleneglycol 20 mass % Diethyleneglycol 10 mass % Surfactant (Surfynol465 prepared by Nisshin 0.1 mass %  Chemical Industry Co., Ltd.)Ion-exchange water 54.9 mass %  

The above respective compositions were mixed, agitated, filtered bymeans of the 1-μm filter, and dark yellow ink as an aqueous pigment inkof the present invention was thus prepared. A mean particle diameter ofthe pigment in the ink concerned was 120 nm, and surface tension γthereof was 36 mN/m.

<Preparation of Dark Magenta Ink> Magenta pigment dispersion 15 mass %Ethyleneglycol 20 mass % Diethyleneglycol 10 mass % Surfactant (Surfynol465 prepared by Nisshin 0.1 mass %  Chemical Industry Co., Ltd.)Ion-exchange water 54.9 mass %  

The above respective compositions were mixed, agitated, filtered bymeans of the 1-μm filter, and dark magenta ink as an aqueous pigment inkof the present invention was thus prepared. A mean particle diameter ofthe pigment in the ink concerned was 113 nm, and surface tension γthereof was 35 mN/m.

<Preparation of Light Magenta Ink> Magenta pigment dispersion  3 mass %Ethyleneglycol 25 mass % Diethyleneglycol 10 mass % Surfactant (Surfynol465 prepared by Nisshin 0.1 mass %  Chemical Industry Co., Ltd.)Ion-exchange water 61.9 mass %  

The above respective compositions were mixed, agitated, filtered bymeans of the 1-μm filter, and light magenta ink as an aqueous pigmentink of the present invention was thus prepared. A mean particle diameterof the pigment in the ink concerned was 110 nm, and surface tension γthereof was 37 mN/m.

<Preparation of Dark Cyan Ink> Cyan pigment dispersion 10 mass %Ethyleneglycol 20 mass % Diethyleneglycol 10 mass % Surfactant (Surfynol465 prepared by Nisshin 0.1 mass %  Chemical Industry Co., Ltd.)Ion-exchange water 59.9 mass % 

The above respective compositions were mixed, agitated, filtered bymeans of the 1-μm filter, and dark cyan ink as an aqueous pigment ink ofthe present invention was thus prepared. A mean particle diameter of thepigment in the ink concerned was 95 nm, and surface tension γthereof was36 mN/m.

<Preparation of Light Cyan Ink> Cyan pigment dispersion 2 mass %Ethyleneglycol 25 mass % Diethyleneglycol 10 mass % Surfactant (Surfynol465 prepared by Nisshin 0.2 mass % Chemical Industry Co., Ltd.)Ion-exchange water 62.8 mass %

The above respective compositions were mixed, agitated, filtered bymeans of the 1-μm filter, and light cyan ink as an aqueous pigment inkof the present invention was thus prepared. A mean particle diameter ofthe pigment in the ink concerned was 92 nm, and surface tension γthereof was 33 mN/m.

<Preparation of Dark Black Ink> Black pigment dispersion 10 mass %Ethyleneglycol 20 mass % Diethyleneglycol 10 mass % Surfactant (Surfynol465 prepared by Nisshin 0.1 mass %  Chemical Industry Co., Ltd.)Ion-exchange water 59.9 mass %  

The above respective compositions were mixed, agitated, filtered bymeans of the 1-μm filter, and dark black ink as an aqueous pigment inkof the present invention was thus prepared. A mean particle diameter ofthe pigment in the ink concerned was 85 nm, and surface tension γthereof was 35 mN/m.

<Preparation of Light Black Ink> Black pigment dispersion 2 mass %Ethyleneglycol 25 mass % Diethyleneglycol 10 mass % Surfactant (Surfynol465 prepared by Nisshin 0.1 mass % Chemical Industry Co., Ltd.)Ion-exchange water 62.9 mass %

The above respective compositions were mixed, agitated, filtered bymeans of the 1-μm filter, and light black ink as an aqueous pigment inkof the present invention was thus prepared. A mean particle diameter ofthe pigment in the ink concerned was 89 nm, and surface tension γthereof was 36 mN/m.

[Colorless Ink]

The colorless ink stands for ink that does not substantially contain thecolor material, and it is preferable that the variation of ΔE of theimage portion, which is caused by the existence of the colorless ink, be3 or less. Content components of the colorless ink either may bedissolved uniformly or may be present in a heterogeneous dispersionsystem. Moreover, though even colorless ink obtained by removing thecolor material from the recording ink for use is usable, it ispreferable to add the following. As addable matter, resin in a dissolvedstate in an aqueous system, resin in a dispersed state in the aqueoussystem, resin in a dissolved state in an organic solvent system, resinin a dispersed state in the organic solvent system and the like can begiven; however, the resin in the dissolved state in the aqueous systemand the resin in the dispersed state in the aqueous system arepreferable.

The resin in the dissolved state in the aqueous system includes, forexample, polyvinyl alcohol, gelatin, polyethylene oxide, polyvinylpyrrolidone, polyacrylic acid, polyacrylamide, polyurethane, dextran,dextrin, carrageenan (κ, l, λ, etc.), agar, pullulan, water-solublepolyvinyl butyral, hydroxyethylcellulose, carboxymethylcellulose, andthe like.

The resin in the dispersed state in the aqueous system is preferablythermoplastic resin, and for example, includes polycarbonate,polyacrylonitrile, polystyrene, polyacrylic acid, polymethacrylic acid,polyvinyl chloride, polyvinylidene chloride, polyvinyl acetate,polyester, polyamide, polyether, copolymers thereof, and salts thereof.Among them, preferable are a styrene-acrylic acid ester copolymer, amethacrylic acid ester-acrylic acid ester copolymer, a vinylchloride-vinyl acetate copolymer, a vinyl chloride-acrylic acid estercopolymer, an ethylene-vinyl acetate copolymer, an ethylene-acrylic acidester copolymer, and SBR latex. For the thermoplastic resin or the fineparticles thereof, plural polymers different in monomer composition,particle diameter and polymerization degree may be mixed together foruse.

Moreover, in the case of mixing the recording ink and the colorless inktogether, it is desirable that an aggregation of the color materialshould not occur substantially, and specifically, that a rate of lightabsorbance change should be less than 5%. As an example of theabove-described case, the recording ink and the colorless ink are mixedtogether on the recording medium P. Moreover, in the case of supplyingthe colorless ink and the recording ink from the inkjet nozzles, therecording medium P is sometimes contaminated by both of the inks thoughthis is not preferable. Furthermore, there is a case of using the samerecording head 22 for the recording ink and the colorless ink for eachimage forming mode. Even in such cases, the lowering of the imagequality and the lowering of the gloss must not occur. When this pointwas studied, it was found that the lowering of the image quality and thelowering of the gloss did not occur in the case where, when therecording ink and the colorless ink were mixed together, the lightabsorbance change was 5% or less with respect to light absorbanceimmediately thereafter. More specifically, 10 ml of the colorless inkwas added to 40 ml of the recording ink, followed by mixing, and lightabsorbance of a supernatant portion immediately thereafter was measured.Next, the above-described mixed solution was hermetically sealed, storedfor 3 days under an environment of 25° C., and the light absorbance ofthe supernatant portion was measured in a similar way. Then, both of themixed solutions were compared with each other. Note that, with regard toeach light absorbance of the ink, the maximum value of the lightabsorbance within a range of 400 nm to 700 nm was measured by using aspectrophotometer (U-3200 made by Hitachi, Ltd.). Then, based on thisvalue, the rate of the light absorbance change was calculated as (lightabsorbance before storage−light absorbance after storage)/(lightabsorbance before storage)×100 (%). For example, the light absorbancechanges of the above-described respective recording inks resulted asfollows: 1.0% in dark yellow ink, 1.3% in dark magenta ink; 0.7% inlight magenta ink; 0.2% in dark cyan ink; 0.5% in light cyan ink; 0.9%in dark black ink; and 1.2% in light black ink.

The ink jetting of the colorless ink is performed by using the recordinghead 22 similar to that for use in the recording ink. A preferredembodiment in this case includes the ink jetting to be performedsimultaneously with the ink jetting of the recording ink. For example,it is desirable that eight recording heads 22 be prepared as describedabove, and be individually used for Y, M, C, K, LC, LM, LK and thecolorless ink, and that the jetting of the colorless ink be performedsimultaneously with the image formation by the recording heads 22.However, in this case, the recording ink and the colorless ink aresometimes mixed together on the recording medium P before being absorbedin the media, and accordingly, a degree of freedom in prescription ofthe recording ink and the colorless ink is lowered. In order to avoidthis, spots of jetting the recording ink and spots of jetting thecolorless ink may be provided separately from each other, and after thejetting of either of the ink is completed, the other one may be jetted.

[Preparation of Colorless Ink]

Here, preparation of the colorless ink will be described by taking aspecific example.

Resin (acrylic ester copolymer: Tg of 75° C., mean particle diameter of0.2 μm) 2.0 mass % as solid content Ethyleneglycol 22.0 mass % Glycerin8.0 mass % Triethyleneglycol monobutylether 5.0 mass % 2-pyrrolidone 2.0mass % Surfynol 465 (prepared by Nisshin Chemical Industry 0.5 mass %Co., Ltd.)

Pure water was added to the above, and the colorless ink was finished toreach 100 mass %.

Next, the inkjet recording method will be described while describing acalculation process of the data for the recording ink by the halftonemodule 206 and a calculation process of the data for the colorless inkby the colorless ink calculation module 207 by referring to FIG. 8. FIG.8 is a flowchart showing an outline of the process of the halftonemodule 206.

As shown in FIG. 8, when the image formation is started, the printerdriver 203 sequentially scans the respective pixels from an upper leftcorner of one image, which is taken as an origin, based on image datainputted from the application program 201, and first creates gradationdata DS (Y, M, C, K, LM, LC and LK, each having 8 bits) after colorcorrection for each pixel in order along the scanning direction of thecarriage 23 from the color gradation correction module 205 (Step S100).

Next, the printer driver 203 performs processing for determining on/offof a recording dot based on the gradation data DS (Step S110), andcreates the gradation data DS of the next pixel (S120). This is repeatedfor all the pixels in a predetermined region (S130).

After the processing is performed for the predetermined region, a totaladhered amount of the recording ink in the region concerned is obtained(S140), and based on this value, the adhered amount of colorless ink tobe placed onto the region concerned is determined (S150). Specifically,the adhered amount of colorless ink is obtained so that the sum total ofthe adhered amounts of recording ink and colorless ink can reach apredetermined amount or more. Then, it is determined as to which pixelposition the colorless ink is to be jetted in the region concerned(S160), and the above-described processing is repeated for allpredetermined regions on the recording surface of the recording medium P(S170). As described above, the jetted positions of the colorless inkare determined for all the predetermined regions on the recordingsurface of the recording medium P. Accordingly, the colorless ink willbe jetted also onto the blank portion onto which the recording ink isnot jetted, and the gloss will be obtained also on the blank portion.

The predetermined region in this case is a unit area on the recordingmedium, on which the sum total of the adhered amounts of colorless inkand recording ink is to be maintained to be a certain amount or more.The minimum of the predetermined region is a one-pixel unit, and themaximum thereof is a full surface of the recording medium. However, itis obvious that, for the purpose of imparting the evenness of the gloss,an effect of defining the predetermined region is small when the fullsurface is taken as the unit. Meanwhile, when the one-pixel unit isdefined as the predetermined region, the adhered amount of colorless inkwill be controlled most finely. However, even if the adhered amount ofcolorless ink on one pixel is assumed to be 0, when the recording inknear a limit of the ink absorbing capacity of the recording medium areplaced on all the pixels in the periphery thereof, it is advantageous inink overflow not to place the colorless ink on the pixel concerned.Hence, a desirable size exists in the size of the unit area.

Specifically, with regard to the size of the unit area, as a result of astudy of the inventor of the present invention, it has been found that,in the case of improving the evenness of the gloss of the recordingsurface, 2 mm or less is essential as the maximum unit to be controlled,and 0.5 mm square is more preferable. Resolving power of a human eye hasthe highest sensitivity at an interval of 0.5 mm when a distance of theeye to the recording medium is set at approximately 30 cm. Hence, in thecase of ensuring evenness of a black density of the recording surface bymeans of dots of the recording head, it is necessary that the dots bedistributed at a spatial frequency higher than the above.

However, it has been found that the resolving power of the human eye isnot very high for characteristics such as the gloss, and that a verylarge feeling of wrongness does not occur even if portions with thegloss and portions without the gloss are distributed uniformly at aninterval of approximately 1 mm. Therefore, if the evenness of the glossis considered, it is preferable that the unit area be 1 mm square, andthat the sum total of the adhered amounts of colorless ink and recordingink within the unit area be set at the predetermined amount or more. Itis desirable that the sum total of the adhered amounts of colorless inkand recording ink in the case of defining the unit area as 1 mm squareas described above be 2 cc/m² or more.

Moreover, a block formed of an aggregate of the n (n>1) pieces of pixelsmay also be defined as the unit area.

In general, in the case of printing an image having gradation, such as aphotograph, by the inkjet mode, the number of gradations for each pixelis short, and accordingly, a halftone process using error diffusion anda dither matrix becomes necessary. In this case, when the area of theunit to be controlled is set as a unit of the dither matrix, data forthe colorless ink can be calculated simultaneously with the halftoneprocess, and this is efficient. In particular, the dither matrix is atechnique for use when a high-speed output is desired though the imagequality is not required very much, where an effect that the calculationof the adhered amount of colorless ink can be made at the high speed islarge. The dither matrix is one taking a×b (=n) pixels in a usual imageas one block and using the block as a unit for determining dotformation, where a is the number of pixels in the lateral direction andb is the number of pixels in the longitudinal direction. Specifically,the unit area of the adhered amounts of colorless ink and recording inkis set as the block formed of an aggregate of the n (n>1) pieces ofpixels, and the adhered amounts of recording ink and colorless ink canbe thus controlled correspondingly to the dither matrix.

Note that, in the case of setting the block formed of the aggregate ofthe n pieces (n>1) of pixels as the unit area, the block of the samepixels as those for the error diffusion is used for creating the datafor the colorless ink; however, without being limited to this, thecreation of the data for the colorless ink may be combined with thehalftone process. Specifically, for example, the error diffusion isperformed at 1080 dpi for a one-pixel unit to determine the jettedpositions of the recording ink, and thereafter, the positions aredivided into four (2×2)-pixel blocks to calculate the adhered amounts ofrecording ink, thus making it possible to determine the jetted positionsof the colorless ink. Meanwhile, a dither process is used as thehalftone process to determine recorded dots within the matrix concernedsimultaneously with the dither process, and thereafter, the sum total ofthe jetted ink within the matrix concerned may be determined so as toreach a predetermined value. Moreover, blocks for the colorless ink maybe made separately from the dither matrix, and the colorless ink may becalculated.

The blocks as the unit area for calculating the colorless ink are notlimited to the above-described 2×2, and may be made to a larger size. Inthis case, the formation of the colorless ink dots can be determinedwhile considering the adhered amount in a wider region. For example, asshown in FIG. 9A and FIG. 9B, when the sum total of the adhered amountsof colorless ink and recording ink is defined as 25% as a predeterminedadhered amount, one dot of the colorless ink will be jetted onto eachfour-pixel block of FIG. 9A. Meanwhile, when the recording ink iscalculated in a region of FIG. 9B, which is wider than the four-pixelblock, it is seen that the sum total of the adhered amounts is 50% evenbefore the colorless ink is jetted. Hence, when the determination ismade based on the 2×2 pixel block, one dot of the colorless ink will beplaced in FIG. 9A, but no dot will be placed in FIG. 9B. Characteristicssuch as the gloss appear as average characteristics of the entire regionhaving some area, and accordingly, even if the colorless ink isdetermined not to be placed in the case of FIG. 9B, an influencetherefrom hardly occurs. In fact, there was no difference in glossbetween the case where the block was formed of 16 (4×4) pixels and theamount of recording ink and colorless ink therein was set at 25% and thecase where the block was formed of 2×2 pixels. It is obvious that thecase of FIG. 9B is more economical since the adhered amount of colorlessink can be reduced more effectively.

Note that, though the square in which longitudinal sides and lateralsides were equal to each other was used as the block of the pixels here,a block shape can be selected appropriately from a rectangle, a rhombusand the like without being limited to the above. Furthermore, it is notnecessary that all the blocks be formed into the same shape with thesame number of pixels, and the shapes of the blocks may differ dependingon a place of the image within a range where a difference between themaximum number and the minimum number does not substantially exceed adouble therebetween. This is similar to a concept of creating a screenpattern in so-called halftone dot formation in printing.

Then, when the calculation processes for the data for the recording inkand the data for the colorless ink are finished as shown in FIG. 8, theimage forming apparatus 200 outputs the data for the recording ink andthe data for the colorless ink to the control circuit 100 of the inkjetprinter 1. Based on the data for the recording ink and the data for thecolorless ink, the control circuit 100 controls the conveyor motor 101,the carriage drive motor 231, the recording heads 22 and the like toconvey the recording medium P, and to jet the recording ink and thecolorless ink from the recording heads 22 while driving the carriage 23.

Thereafter, when the recording medium P on which the image is formed isconveyed to the fixing unit 4, the control circuit 100 allows the heatsource 43 to generate heat to reach a predetermined heating temperaturebased on a detection result of the temperature sensor 413 whilecontrolling the heating roller drive motor 44 so that the heating roller41 can convey the recording medium P. In such a way, the image is fixedonto the recording medium P. Then, in this case, the recording ink, thecolorless ink and the thermoplastic resin on the recording medium P aremelted and turned into a coating film, and accordingly, the gloss isimproved.

Note that, though the method of fixing the image by heating is appliedas the fixing process in this embodiment, the image may also be fixed bypressurization as the fixing process, and further, the image may also befixed by both the heating and the pressurization.

The fixing process may be performed continuously with the imageformation after the image is formed, and may be performed collectivelyafter a certain amount of images are formed. In the present invention,it is preferable to implement the fixing process within a range of afixed time after the image formation and the jetting of the colorlessink from a viewpoint of the color reproduction. It is preferable toimplement the fixing process within 5 seconds or more to 10 minutes orless after the image formation and the jetting of the colorless ink, andmore preferable to implement the fixing process in 10 seconds or more to5 minutes or less.

In the above-described method, it is particularly preferable to performthe fixing process with heat for the image in which the inorganicpigment and the thermoplastic resin exist mixedly or in which both ofthem exist adjacent to each other. In this case, it is particularlypreferable to partially or completely melt the thermoplastic resin, andfurther to turn the thermoplastic resin into the coating film.

For the fixing process with heat, only energy enough to exert the effectof the present invention sufficiently is required. When higher energymore than necessary is given, deformation and the like of the supportoccur to rather deteriorate the glossiness, and accordingly, this is notpreferable. Heating temperature just needs to be temperature at whichthe image can be smoothened, and preferably within a range of 60 to 200°C., and more preferably within a range of 80 to 160° C.

The heating performed in the fixing process may be performed either by aheating apparatus built in the inkjet printer 1, such as the fixing unit4 illustrated in this embodiment, or by a heating apparatus providedseparately. It is preferable to use, as the heating apparatus, a heatingroller, a heating belt, or a system formed by combining these becausethese devices eliminate an occurrence of unevenness, are small in termsof space, and are suitable for performing continuous processing.Moreover, for these heating apparatuses, a fixing machine with heat foran electronic photograph can be diverted, and these heating apparatusesare advantageous in terms of cost.

Moreover, as a fixing machine for fixing the image by implementing boththe heating and the pressurization, for example, one for implementingthe heating and pressurization processes by passing the recording mediumbetween a heating roller building a heating element therein and apressing roller, one for implementing the heating and the pressurizationby sandwiching the recording medium between two heating rollers, and thelike are given.

Conveying speed of the recording medium in the case of using the heatingroller is preferably in a range of 1 to 15 mm/second. This is preferablefrom a viewpoint of the image quality as well as a viewpoint ofhigh-speed processability. In order to obtain higher texture and gloss,it is preferable to perform the pressurization simultaneously with theheating, or immediately thereafter. As pressure for the pressurization,a range of 9.8×10⁴ to 4.9×10⁶ Pa is preferable. This is because thepressurization promotes growth of the coating film.

As described above, according to the inkjet recording method of thisembodiment, the recording ink is jetted onto the recording medium P, thecolorless ink for improving the gloss is jetted onto the recordingmedium P, and the image is thus formed. Accordingly, even if the glossof the image-formed regions is improved by the color materials containedin the recording ink, the colorless ink is jetted onto the blank portionand the highlighted portion in which the adhered amount of recording inkis small, thus making it possible to improve the gloss of theseportions. In particular, the adhered amount of colorless ink per unitarea is determined in response to the adhered amount of recording inkper unit area. Accordingly, the gloss on the recording surface of therecording medium can be made even, and the feeling of wrongness owing tothe unevenness of the gloss can be solved.

Note that, naturally, the present invention is not limited to theabove-described first embodiment, and is modifiable as appropriate.

For example, in the first embodiment, the method of making thecalculation so that the sum total of the adhered amounts of recordingink and colorless ink per unit area can be a predetermined amount ormore in the case of determining the adhered amount of colorless ink perunit area is illustrated. However, besides this, also used may be amethod of making a calculation so that the jetted amount of colorlessink can become larger in a region where the adhered amount of recordingink per unit area is a predetermined amount or less than in a regionwhere the adhered amount of recording ink is more than the predeterminedamount.

Moreover, the first embodiment has a configuration in which the imageforming apparatus 200 performs the process of the data for the colorlessink and the halftone process as a recording ink dot process. However,both of the processes may be performed by another apparatus, and may beperformed individually by different apparatuses. A merit of separatelyperforming the process of the data for the colorless ink and thehalftone process as the recording ink dot process is that the halftoneprocess is enabled to be processed by the printer driver of the imageforming apparatus 200, a host personal computer and the like, and thatthe calculation for the colorless ink can be loaded in the inside of theinkjet printer 1. The halftone process is a process requiring a longtime, and accordingly, it is frequent that a result of a calculationperformed once is stored in a file, and later used repeatedly foroutputs. Characteristics such as the gloss are sometimes affected by theambient temperature and humidity, and when a file reflecting thetemperature and the humidity at a certain point of time is created, thefile becomes unusable for the output in a different temperature andhumidity environment later on. In such a case, when the process ofdetermining the amount of colorless ink is loaded in the inside of theprinter, the same printing file becomes easily and repeatedly usable.

Second Embodiment

A second embodiment according to the present invention will be describedbelow. In the above-described first embodiment, the inkjet recordingmethod of determining the adhered amount of colorless ink per unit areain response to the adhered amount of recording ink per unit area hasbeen described. In this second embodiment, description will be made ofan inkjet recording method of determining the adhered amount ofcolorless ink per unit area in response to the adhered amount ofrecording ink per unit area, and further determining the jettedpositions of the colorless ink in response to the jetted positions ofthe recording ink. Note that, in the description below, the samereference numerals will be assigned to the same portions as those of thefirst embodiment, and description thereof will be omitted.

FIG. 10 is a flowchart showing an outline of a process of the halftonemodule 206 according to the second embodiment. Referring to FIG. 10,description will be made of a calculation process of the data for therecording ink by the halftone module 206 and a calculation process ofthe data for the colorless ink by the colorless ink calculation module207.

As shown in FIG. 10, when the image formation is started, the printerdriver 203 sequentially scans the respective pixels from the upper leftcorner of one image, which is taken as an origin, based on the imagedata inputted from the application program 201, and first creates thegradation data DS (Y, M, C, K, LM, LC and LK, each being an 8-bit value)after the color correction for each pixel in order along the scanningdirection of the carriage 23 from the color gradation correction module205 (Step S200).

Next, the printer driver 203 performs the processing for determiningon/off of the recording dot based on the gradation data DS (Step S210),and creates the gradation data DS of the next pixel (S220). This isrepeated for all the pixels in the unit area (S230).

After the processing is performed for the unit area, a total adheredamount of the recording ink in a region concerned is obtained (S240),and based on this value, the adhered amount of colorless ink to beplaced onto the region concerned is determined (S250). Specifically, theadhered amount of colorless ink is obtained so that the sum total of theadhered amounts of recording ink and colorless ink can reach apredetermined amount or more. Then, it is calculated as to how largenumber of dots in the unit area the colorless ink is to be adhered ontobased on the determined adhered amount of colorless ink (S260).

Subsequently, the printer driver 203 checks dot positions serving as thejetted positions of the recording ink (Step S270), and determines dotpositions serving as the jetted positions of the colorless ink inresponse to the jetted positions of the recording ink and the number ofdots of the colorless ink (Step S280). For example, the jetted positionsof the colorless ink are determined so that the jetted positions of therecording ink and the jetted positions of the colorless ink in the unitarea cannot become positions adjacent to or overlapped on each other. Aspecific example of the above will be described with reference to FIGS.11 to 14. In this specific example, a four (2×2)-pixel block is used asthe unit area. Note that, in FIG. 11 to FIG. 14, a square painted blackis a jetted position of the recording ink, a square painted light is ajetted position of the colorless ink, and a non-painted square is aposition without ink adhered thereto.

In FIG. 11 to FIG. 13, the sum total of the adhered amounts of colorlessink and recording ink is set at 25%. Accordingly, when the recording inkis not jetted onto the four-pixel block, as shown in FIG. 1C, one dot ofthe colorless ink is jetted onto a lower left pixel in the four-pixelblock, and when the recording ink is jetted onto the four-pixel block,the colorless ink is not jetted onto the four-pixel block. In such away, the jetted positions of the recording ink and the jetted positionof the colorless ink are prevented from being overlapped on each other.Then, for example, when the recording ink is jetted onto jettedpositions as shown in FIG. 11A, one dot of the colorless ink is jettedonto a four-pixel block onto which the recording ink is not jetted amongthe respective four-pixel blocks. In this case, when the colorless inkis jetted as shown in FIG. 1C, the colorless ink concerned sometimesbecomes adjacent to a jetted position of the recording ink in anadjacent four-pixel block. Accordingly, the printer driver 203 refers tothe jetted positions of the recording ink, and when the jetted positionof the colorless ink becomes adjacent to upper, lower, left and rightside thereof, varies the jetted position of the colorless ink from thelower left so as not to be adjacent to the jetted position of therecording ink, thereby determining the jetted position of the colorlessink (refer to FIG. 11B). Here, when the jetted position of the colorlessink becomes a position diagonal to the jetted position of the recordingink, this may be permitted since the overlap of both is small.

FIG. 12 is illustrations of the case where the jetted positions of therecording ink are less than those of FIG. 11 (refer to FIG. 12A). Alsoin FIG. 12, as in the case of FIG. 11, the jetted positions of therecording ink and the jetted positions of the colorless ink are adaptedto be neither overlapped on each other nor adjacent to each other (referto FIG. 12B).

FIG. 13 is illustrations of the case where the jetted positions of therecording ink are linearly arrayed (refer to FIG. 13A). In such a linearpattern and the like, a bleeding of the dot is prone to be visuallyrecognized, and accordingly, the jetted positions of the colorless inkare determined so as not to be adjacent to the recording ink as shown inFIG. 13B.

FIG. 14 is illustrations of the case where the sum total of the adheredamounts of colorless ink and recording ink are set at 50%. When therecording ink is not jetted onto the four-pixel block, as shown in FIG.14C, only one dot of the colorless ink is jetted onto each of two pixelsin the four-block pixel, which are arranged on a diagonal line. Here,while two types of arrangement patterns are illustrated in FIG. 14C,when the recording ink is jetted onto an adjacent four-pixel block, thearrangement pattern in which the pixels are not adjacent to the jettedpositions of the recording ink concerned is selected. Meanwhile, whenthe recording ink is jetted onto the four-pixel block concerned, onlyone dot of the colorless ink is jetted onto one pixel located on adiagonal line thereof. Then, for example, when the recording is jettedonto jetted positions as shown in FIG. 14A, the colorless ink is jettedonto the jetted positions as shown in FIG. 14B.

Then, as shown in FIG. 10, the above-described processing is repeatedfor all the predetermined regions on the recording surface of therecording medium P (S290). As described above, the jetted positions ofthe colorless ink are determined for all the predetermined regions onthe recording surface of the recording medium P, and accordingly, thecolorless ink will be jetted also on the blank portion on which therecording ink is not jetted, and the gloss will be obtained also on theblank portion.

EXAMPLE Example 1

In Example 1, an error diffusion method was used as the halftoneprocess, and with regard to the calculation of the adhered amount ofcolorless ink, the total amount of recording ink and colorless ink wasset to not less than the predetermined amount on a four-pixel unit madeof two pixels in the longitudinal direction and two pixels in thelateral direction as one block. Recording conditions are as follows.

Recording resolution: Main scanning/sub-scanning 1080 dpi

Ink type: Dark and light cyan, dark and light magenta, dark and lightblack, yellow, and colorless ink

(eight in total)

Ink droplet: 6.7 μl

Main scanning speed: 705 mm/sec

Error diffusion resolution: 540 dpi

Number of error diffusion levels: 7

Ink control block (unit area): Longitudinally and

laterally two-by-two four pixels

Fixing temperature: 100° C.

Fixing pressure: 4 kg/cm²

Fixing time: 1.0 second

To gradation levels of 0 to 6 calculated by performing an errordiffusion process as the halftone process by values obtained by dividing540 dpi by seven, dark and light dots were assigned as shown in FIG.15A. However, Y does not have a light color but has only a dark color,and accordingly, the light dots in the drawing were defined as the darkcolors. Hence, the levels 4 to 6 come to have completely the samepattern, a multi-level processing of five levels of 0 to 4 wasperformed. With regard to the image data, patches of 33 gray gradations(in which data value are 0, 8, 16, 24 . . . , 248, 255) were defined asmixtures (so-called composite blacks) of the gray with all the recordinginks of C, M, Y, K, LM, LC and LK, and the image was thus formed. Withregard to the patches, gradation correction was made in advance for theimage data so that a lightness (L* value) could be uniform from 0 (noink) to 255 (the darkest patch), and amounts of the respective recordinginks were balanced so than an a* value and a b* value could besubstantially 0. A size of each patch was set at 4 cm square so as tomake it possible to measure the gloss and a C value.

FIG. 16 shows a graph in which the ink amount of the patch of eachgradation level was plotted for each ink. A percent value of an axis ofordinates is a ratio of the adhered amount of ink to the medium, wherethe case of placing any one of the ink droplets onto all the pixels wasdefined as 100%. In this case, the ink droplets become 6.7 μl for apixel area of 23.5×23.5 μm² and the adhered amount becomes 12.1 cc/m².

With respect to this, some types of images in each of which thecolorless ink dots were formed on the recording surface of the recordingmedium by jetting the colorless ink from the recording head 22 for thecolorless ink simultaneously with usual recording were created whilechanging the amount of colorless ink.

The sum total of the colorless ink and recording ink was changed from12.5 to 100%. As shown in FIG. 17A, in the case of setting the sum totalat 25%, when the recording ink is not present at all in the block offour pixels, the colorless ink is jetted onto an upper left pixel. Whenat least one droplet of any recording ink is placed on the block, thecolorless ink is not placed. Specifically, it is only a block of whichgradation level was calculated to be 0 that the colorless ink is to beplaced thereonto. The adhered amount in this case becomes 3.0 cc/m².

In the case of setting the sum total at 50%, as shown in FIG. 17B, thecolorless ink dots are jetted so that at least two droplets of the dotsof the recording ink and the colorless ink can be placed onto eachblock. When two droplets of the recording ink are placed, the colorlessink dot is not formed. In the block of which gradation level wascalculated to be 0, the colorless ink dots are formed on upper left andlower right pixels. In a block of which gradation level was calculatedto be 1, the colorless ink dot is formed at a pixel position located ona diagonal line with respect to the recording ink. The adhered amount inthis case becomes 6.1 cc/m².

In the case of setting the sum total at 75%, as shown in FIG. 17C, thecolorless ink dots are jetted so that at least three droplets of therecording ink and the colorless ink are placed onto the block. Whenthree droplets are placed, the colorless ink dot is not formed. Theadhered amount in this case becomes 9.1 cc/m².

In the case of setting the sum total at 100%, at least one droplet ofthe recording ink or the colorless ink dot is adapted to be formed onevery pixel without fail. Specifically, when the recording ink is jettedonto each pixel, the colorless ink is not jetted.

An image in which the sum total was set at 12.5% was created by formingat least one droplet of the recording ink or the colorless ink onto aunit of two blocks, that is, eight pixels, and a 60-degree speculargloss of an image-formed surface thereof was measured according toJIS-Z-8741. For the measurement, a variable angle gloss system(VGS-1001DP) made by Nippon Denshoku Industries Co., Ltd. was used.Gloss measurement values of the patches in which the sum totals were 25to 100% are shown in FIG. 18.

In the sum total of 25%, though an improvement of the gloss wasrecognized, a lowering of the gloss on the highlighted portion was ableto be recognized by visual evaluation in comparison with other portions.In the sum total of 12.5%, the unevenness of the gloss was apparent bythe visual evaluation, and the image concerned was determined to be nogood. From the above, it is understood that surface coverage of therecording medium by the dots is suitably 25% or more, which is 3 cc/m²or more in conversion to the amount of droplets. In the sum totals of12.5% (1.5 cc/m²), the improvement effect is not sufficient.

Moreover, in the case of always setting the sum total of the adheredamounts of recording ink and colorless ink at 100% or more, it is notnecessary to make the blocks for the calculation of the colorless ink,and the colorless ink can be always placed onto the pixels on which therecording ink is not present. According to this method, the calculationof the colorless ink is simplified, thus enabling to shorten acalculation time and simplification of the instrument.

Example 2

In Example 2, the error diffusion method was used for both the halftoneprocess and the calculation of the colorless ink amount. The recordinginks for use were defined to be only dark colors which were four, andrecoding conditions are as follows.

Recording resolution: Main scanning/sub-scanning 1080 dpi

Ink type: Cyan, magenta, black, yellow, and colorless ink (five intotal)

Ink droplet: 6.7 pl

Main scanning speed: 705 mm/sec

Error diffusion resolution: 1080 dpi

Number of error diffusion levels: 2

The calculation of the colorless ink in this case was made based on theimage data before the halftone process was performed for the recordingink. When the sum of CMYK data (8-bit value) is taken, a value thereofbecomes the sum total of the recording inks. For example, when the datafor the colorless ink is a (0 to 255), the sum total of the adheredamounts of colorless ink and recording ink is b, and the adhered amountsof the respective recording inks are Y, M, C and K, the sum total of therecording inks can be calculated by a formula of:a=255×(b/100)−(Y+M+C+K).

In the case of desiring to record the colorless ink at 50%, the data forthe colorless ink is set at 128 at the maximum, and the usual halftoneprocess is performed. Then, as a result, the colorless ink dots areformed on 50% of the recording medium. When the sum total of the adheredamounts of ink on the respective pixel positions is subtracted from 128,the colorless ink is reduced by the amount of recording ink, and as aresult, the sum of the recording ink and colorless ink becomes 50% ormore in the full recording region.

The sum total b of the adhered amounts of recording ink and colorlessink can be set arbitrarily. When the sum total b is larger than 255, thecolorless ink will be placed onto the same pixel position twice or more.This can be easily realized even if the number of colorless ink nozzlesis the same as the number of recording ink nozzles because, in thehigh-quality printing mode, it is common to perform overlap printing ofmaking the recording while thinning the main scanning in order toimprove usual image quality.

When such a configuration is adopted, the halftone process and thecolorless ink calculation can be processed by the same algorithm, andaccordingly, the need for preparing an algorithm separately from thatfor the colorless ink can be saved. In particular, in the case ofpackaging all the processes in the printer, cost of the apparatus can bereduced, and accordingly, this is effective.

As the image data, 16 gradation patches (data value of 0, 16, 32 . . . ,240 and 255) of yellow were printed only by means of the Y ink. Themaximum value of yellow was limited to 75% in advance. A size of eachpatch was set at 4 cm square so as to make it possible to measure thegloss values in a similar way to Example 1. FIG. 19 shows a graph inwhich the ink amount of the patch of each gradation level is plotted.Moreover, FIG. 20 shows a graph showing 60-degree gloss values when thecolorless ink is changed from 25 to 100%. As shown in the graphs, aneffect of improving the gloss, which is accompanied by an increase ofthe ink amount of colorless ink, is observed as in Example 1. Moreover,when the amount of colorless ink is 50% (6 cc/m²) or more as shown inFIG. 20, the effect of improving the gloss with respect to the adheredamount is small. Then, when the amount of colorless ink was set at 13cc/m² or more, in such a pattern in which a blue or black thin line waspresent on a white base, a phenomenon that width of the line was blurredand widened by the colorless ink was observed. Specifically, from aneconomical viewpoint and an influence given to the image quality, therewas a suitable value for the sum total of the adhered amounts in theunit area, and 2 cc/m² or more to less than 13 cc/m² was the optimum.

Example 3

In Example 3, a suitable size of the calculation block of the colorlessink was studied. The sum total of the adhered amounts of colorless inkand recording ink was set at 25% and 50% or more, and visual evaluationwas performed while changing the size of each block as shown in FIG. 21Aand FIG. 21B. Here, 2M×2M pieces of pixels were used as the blocks, andin the setting at 25%, as shown in FIG. 21A, the colorless ink is formedon all the pixels of an upper left block thereof. In the setting at 50%,as shown in FIG. 21B, the colorless ink dots are formed on all thepixels of a lower left block thereof. Length of the block becomes2M×23.5 μm assuming that one pixel is equivalent to 1080 dpi. Patches inwhich this length was changed from 0.09 mm (four-pixel block) to 4.7 mm(200-pixel block) were prepared. In this case, the recording ink was notplaced on the medium at all, but only the colorless ink was placedthereon. Evaluation results are shown in Table 1. TABLE 1 25% 50% M 2 MLength(mm) Evaluation Evaluation  2 4 0.09 ◯ ◯  4 8 0.19 ◯ ◯ 10 20 0.47Δ ◯ 20 40 0.94 Δ Δ 30 60 1.41 X Δ 40 80 1.88 X X 45 90 2.12 X X 50 1002.35 X X 100  200 4.70 X X

From the results, it was understood that the size of the block forcontrolling the colorless ink was suitably set at 1 mm or less. In thecase of setting the size at 0.94 mm, though the unevenness of the glosswas slightly recognized, the texture like so-called matte gloss appearson the contrary. Accordingly, the above-described setting can besuitably used depending on preferences of a user.

1. An inkjet recording method comprising: jetting recording inkcontaining a color material onto a recording medium by a recording head,and colorless ink for improving gloss onto the recording medium by therecording head, to perform image formation; and determining an adheredamount of the colorless ink per unit area in response to an adheredamount of the recording ink per unit area.
 2. The inkjet recordingmethod of claim 1, wherein a jetted position of the colorless ink isdetermined in response to a jetted position of the recording ink.
 3. Theinkjet recording method of claim 2, wherein the jetted position of thecolorless ink is determined preferentially from a position that is notadjacent to or overlapped on the jetted position of the recording ink.4. The inkjet recording method of claim 1, wherein the adhered amount ofcolorless ink is increased in a region where the adhered amount ofrecording ink is a predetermined amount or less than in a region wherethe adhered amount of recording ink is more than the predeterminedamount.
 5. The inkjet recording method of claim 1, wherein the unit areafor the adhered amounts of the colorless ink and the recording ink isset at 1 mm square or less, and a sum total of the adhered amounts ofthe colorless ink and the recording ink in the unit area is set at apredetermined amount or more.
 6. The inkjet recording method of claim 5,wherein the sum total of the adhered amounts of the colorless ink andthe recording ink in the unit area is 2 cc/m² or more.
 7. The inkjetrecording method of claim 6, wherein the sum total of the adheredamounts of the colorless ink and the recording ink in the unit area isless than 13 cc/m².
 8. The inkjet recording method of claim 5, whereinthe unit area for the adhered amounts of the colorless ink and therecording ink is set as a block formed of an aggregate of n (n>1) piecesof pixels.
 9. The inkjet recording method of claim 8, wherein a jettedposition of the colorless ink jetted onto the block is determinedpreferentially from a pixel in which the adhered amount of the recordingink is smaller.
 10. The inkjet recording method of claim 1, wherein theunit area for the adhered amounts of the colorless ink and the recordingink is defined as one pixel, and a sum total of the adhered amounts ofthe colorless ink and the recording ink in the unit area is set at apredetermined amount or more.
 11. The inkjet recording method of claim1, wherein the recording ink contains fine particles.
 12. The inkjetrecording method of claim 1, wherein the recording medium includes amicro-porous recording medium.
 13. The inkjet recording method of claim1, wherein a surface layer of the recording medium contains athermoplastic resin.
 14. The inkjet recording method of claim 13,wherein a fixing process including heating or pressurization isimplemented for the recording medium on which the recording ink and thecolorless ink are jetted.
 15. The inkjet recording method of claim 1,wherein a rate of light absorbance change in mixing the recording inkand the colorless ink with each other is less than 5%.
 16. An inkjetrecording method comprising: jetting recording ink containing a colormaterial onto a recording medium by a recording head, and a colorlessink for improving gloss onto the recording medium by the recording head,to perform image formation, wherein a rate of light absorbance change inmixing the recording ink and the colorless ink with each other is lessthan 5%.
 17. An inkjet printer, comprising: an image forming unit to jetrecording ink containing a color material onto a recording medium by arecording head, and jet colorless ink for improving gloss onto therecording medium by the recording head, thereby performing imageformation; and a control unit to control the image forming unit, whereinthe control unit determines an adhered amount of the colorless ink perunit area in response to an adhered amount of the recording ink per unitarea.
 18. The inkjet printer of claim 17, wherein the control unitdetermines a jetted position of the colorless ink in response to ajetted position of the recording ink.
 19. The inkjet printer of claim18, wherein the control unit determines the jetted position of thecolorless ink preferentially from a position that is not adjacent to oroverlapped on the jetted position of the recording ink.
 20. The inkjetprinter of claim 17, wherein the control unit increases the adheredamount of the colorless ink in a region where the adhered amount of therecording ink is a predetermined amount or less than in a region wherethe adhered amount of the recording ink is more than the predeterminedamount.
 21. The inkjet printer of claim 17, wherein the control unitsets the unit area for the adhered amounts of the colorless ink and therecording ink at 1 mm square or less, and sets a sum total of theadhered amounts of the colorless ink and the recording ink in the unitarea at a predetermined amount or more.
 22. The inkjet printer of claim21, wherein the control unit sets the sum total of the adhered amountsof the colorless ink and the recording ink in the unit area at 2 cc/m²or more.
 23. The inkjet printer of claim 22, wherein the control unitsets the sum total of the adhered amounts of the colorless ink and therecording ink in the unit area at less than 13 cc/m².
 24. The inkjetprinter of claim 21, wherein the control unit sets the unit area for theadhered amounts of the colorless ink and the recording ink as a blockformed of an aggregate of n (n>1) pieces of pixels.
 25. The inkjetprinter of claim 24, wherein the control unit determines a jettedposition of the colorless ink jetted onto the block preferentially froma pixel in which the adhered amount of the recording ink is smaller. 26.The inkjet printer of claim 17, wherein the control unit defines theunit area for the adhered amounts of the colorless ink and the recordingink as one pixel, and sets a sum total of the adhered amounts of thecolorless ink and the recording ink in the unit area at a predeterminedamount or more.
 27. The inkjet printer of claim 17, wherein therecording ink contains fine particles.
 28. The inkjet printer of claim17, wherein the recording medium includes a micro-porous recordingmedium.
 29. The inkjet printer of claim 17, wherein a surface layer ofthe recording medium contains thermoplastic resin.
 30. The inkjetprinter of claim 29, wherein a fixing process including heating orpressurization is implemented for the recording medium on which therecording ink and the colorless ink are jetted.
 31. The inkjet printerof claim 17, wherein a rate of light absorbance change in mixing therecording ink and the colorless ink with each other is less than 5%. 32.An inkjet printer, comprising: an image forming unit to jet recordingink containing a color material onto a recording medium by a recordinghead, and jet colorless ink for improving gloss onto the recordingmedium by the recording head, thereby performing image formation; and acontrol unit to control the image forming unit, wherein a rate of lightabsorbance change in mixing the recording ink and the colorless ink witheach other is less than 5%.